There have been many conflicts during Critical Mass events since the founding of the worldwide bicycling advocacy event in 1992. The conflicts have resulted in injuries, property damage, and arrests, and both bicyclists and motorized vehicle drivers have been victims. Critics say that Critical Mass, held primarily in large metropolitan cities, is a deliberate attempt to obstruct automotive traffic and disrupt normal city functions, when individuals taking part refuse to obey traffic laws, while participants variously consider it a celebration of cycling, of cyclists' rights, and a practical re-imagining of urban space. The event is large, occurs monthly or weekly, and happens in over 300 cities.

Berkeley, California, US On May 11, 2007, an incident occurred in Berkeley, California, when an elderly motorist stopped at an intersection with dozens of bicycles crossing. Activists claim that the driver shouted, while bicyclists were in the intersection, "I'm sick of you people!" Police have not determined who had the right-of-way. The driver attempted to drive through the intersection. The motorist and his wife, two witnesses, and the police all claimed that the cyclists threw their bicycles under the vehicle. This was disputed by other witnesses. Critical Mass participants then rocked the vehicle, pounded the hood, and broke the windshield while verbally abusing the elderly couple. Approximately $3,000 worth of damage was done to the bicycles. Berkeley police did not make any arrests in the incident.

Buenos Aires, Argentina On March 28, 2013, in Buenos Aires, Argentina, a taxi driver ran over a group of Critical Mass riders, injuring 2 cyclists, before fleeing from the scene.

Buffalo, New York, US On May 30, 2003, in Buffalo, New York, during an incident known locally as "Critical Massacre," police stopped two cyclists for "failure to yield to an emergency vehicle." Several people were allegedly attacked by police. Nine cyclists were arrested and three were later convicted, including a journalist.

Chicago, Illinois, US

Chicago Police Department officers are often seen riding with Critical Mass participants, and squad cars block intersections to provide safe passage to Critical Mass cyclists. However, on August 31, 2007, seven riders were arrested on charges of obstructing traffic and disobeying police, and were held overnight. According to some of those arrested, they were released late at night or early in the morning. On multiple occasions, Critical Mass participants attempted to ride on Lake Shore Drive, a road off limits to cyclists. Police prevented participants riding on Lake Shore Drive, by blocking entrance ramps with squad cars when Critical Mass riders approached the road.

Honolulu, Hawaii, US At dusk on March 28, 2008, in Honolulu, Hawaii, police collided with a young woman on a bicycle while trying to stop another bicyclist for traffic infractions. The young woman fell and hit her head on the ground, sustaining injuries requiring hospitalization. While no arrests were made, the police did issue citations and confiscated bicycles.David, Mari-Ela. Police Step Up Patrols at Critical Mass. Hawaii News Now. 26 April 2008.

Houston, Texas, US Due to the gradual increase of cyclists participating in Critical Mass, as well as complaints from motorists and concerns for safety, Houston Police Department officers helped direct the October 2013 Critical Mass ride in Houston, Texas by controlling intersections on the bike route. In January 2014, after two months of providing free security and safe passage through heavy intersections, the Houston Police Department began considering options to charge the group for providing security for the event, citing similarities to providing security for funeral processions through the city limits. Some Critical Mass riders have spoken out in protest of paying a fee to utilize extra security and escorting through busy intersections, while many others support paying fees for extra security to keep the ride safe, which has led to increased discussions between the city and group for additional alternatives to the building conflicts.

London, United Kingdom

In London in September 2005, a few weeks after the 7 July 2005 bombings, Metropolitan Police required the organisers to provide a route six days before the event and placed strict restrictions on riders, under threat of arrest. The threat was retracted when politicians and cyclist groups objected. In October 2005, the ride had close to 1,200 participants, significantly more than usual. A long stop in Parliament Square, part of the government's exclusion area in the Serious Organised Crime and Police Act 2005, led to a slow and cumbersome ride.

One participant sought a declaration from the High Court of England and Wales that police need not be notified about the rides, in a "friendly action" in which neither side sought damages. The High Court agreed, exempting Critical Mass from notification under Section 11 of the Public Order Act 1986. The ruling was reversed on appeal. In 2008, Friends of the Earth, who supported the legal action, said the case would be appealed to England's highest legal authority, the House of Lords, on the grounds that, after 11 years, Critical Mass is "commonly or customarily held." In October 2008, the House of Lords ruled in favour of the Critical Mass participant.

During the 2012 Summer Olympics, cyclists were arrested on opening day after police claim they ignored regulations in place during the Olympics. Jenny Jones of the Green Party and the Greater London Assembly has questioned the police actions, calling them "out of all proportion to common sense" and "tragically unnecessary." A Critical Mass website asked riders to be "peacefully assertive" during the monthly ride, while police claim they asked riders to keep to the south of the River Thames. When the riders got close to the Olympic Park, police began to cordon off the area and arrest riders. In total, 182 people were arrested, of whom 4 were charged, and 178 were bailed pending further inquiries. The action led to 5 of the 182 people arrested being convicted in court.

Minneapolis, Minnesota, US On the August 31, 2007, ride in Minneapolis, Minnesota, a confrontation occurred between cyclists and the police. The police presence included undercover officers, three marked squad cars, a state patrol helicopter, and unmarked vehicles. The ride had been linked with weekend protests of the following year's Republican National Convention. After the arrest of a cyclist for "riding in a snake-like manner," cyclists began chanting "Let him go!" and "What's the charge?" The police called for backup, and dozens of police officers responded, using mace and tasers. Minneapolis police arrested 19 participants, including three minors. The adults were arrested on suspicion of rioting, a gross misdemeanor.

Minsk, Belarus On May 27, 2011, in Minsk, Belarus, police forces aided by special units transported 13 cyclists to the police station for violation of traffic rules and for not following orders to disband. Detained participants were fingerprinted and fined, then released.

On April 30, 2016, two cyclists were arrested during a Critical Mass action. As of September 2016, one was released and the other is facing trial.

New York City, New York, US Police in New York City have claimed that Critical Mass bicyclists blocking intersections to allow bikes to pass may delay emergency vehicles in the gridlock.

2004 Republican National Convention During the US 2004 Republican National Convention, police arrested 250 riders after the ride caused "massive disruptions" in the city. Many court cases resulted regarding the legality of the ride, whether police have the right to arrest cyclists and seize bicycles, and whether the event needs a permit. In December 2004, a federal judge dismissed New York City's injunction against Critical Mass as a "political event." On March 23, 2005, the city filed a lawsuit seeking to prevent Time's Up!, a direct action environmental group, from promoting or advertising Critical Mass rides. The lawsuit also stated that Time's Up! and the public could not ride or gather at a Critical Mass bike ride, claiming a permit was required.

2008 bicycle rally During a bicycle rally on July 25, 2008, NYPD patrolman Patrick Pogan pushed rider Christopher Long off his bicycle. In a criminal complaint, Pogan wrote that he had ordered Long to stop because he was weaving in and out of traffic, forcing vehicles to swerve or stop, and generally disrupting the normal flow of traffic. In addition, Pogan wrote that he suffered lacerations on his arms because Long steered his bike into him and knocked him down, and when he tried to place Long under arrest, Long began flailing, kicking and screaming, "You are pawns in the game!" Long spent the next day in police custody on charges of attempted assault, resisting arrest and disorderly conduct.

Within days, members of the rally's sponsoring organization Time's Up! posted a video of the incident on YouTube. The video went viral and received over 400,000 hits within five days. Critical Mass activists claimed it supported their claims that the police department has demonstrated a pattern of arresting participants in the rally on false charges. Witnesses also disputed Pogan's account, saying that Long was the one to receive injuries, traffic was stopped for the rally, and that Pogan had simply scanned the group of cyclists to find one he could take down.

The union that represents NYPD officers said Pogan was just doing his job to protect the public from a reckless bike rider, and Pogan's father — himself a retired NYPD detective — defended him, saying "You gotta do what you gotta do to make an arrest." The prosecutor's office dismissed the charges against Long. NYPD placed Pogan, who had spent only three weeks as a police officer, on a desk assignment while the city investigated the incident.

On December 16, 2008, Pogan appeared in court and pleaded not guilty to felony charges of falsifying business records and filing a false instrument, and misdemeanor charges of third-degree assault, second-degree harassment and making a punishable false written statement. After the indictment, the police department suspended Pogan and, two months later, Pogan resigned as the department prepared to fire him. His attorney said that his defense would center on the department's training procedures and claims that events occurring off-camera needed to be taken into account.

On July 8, 2009, it was reported that Long, then a Hoboken, NJ, resident, was suing the New York Police Department for $1.5 million, alleging that Pogan falsified his arrest report in order to legitimize his assault on Long.

During his trial in April 2010, Pogan acknowledged that the video looked "very extreme." He testified that he had anticipated a collision with Long, since the rider lowered his right shoulder as the officer approached. Jurors found Pogan not guilty of harassment and assault and acquitted him of four of the seven counts of which he had been charged. He was found guilty of filing a criminal complaint that contained false statements concerning the cyclist. Pogan received no jail time but, because he is a convicted felon, he is not eligible to become a New York City police officer in the future. Long said in an interview that he was pleased with the verdict, in part because it would prevent Pogan from becoming a New York City police officer again.

Oakland, California, US