On May 1st, 2008, several participants were arrested by the Riga, Latvia Police Department because of "breaking the traffic rules and disobeying the police". However, some of the arrested people claim they weren't told by the policemen what they are accused for.
Eyewitnesses are certifying that at least three cases 1 May 2008 Riga Police used to drive vehicle consciously over the bicyclists, brutally pushed cyclists by dresses while driving to fall over head (at full speed), and at last, arrested two AFI press photographers for the reason they just filmed police brutality. Camera content was demanded to erase. Due the unreasonable low interest on such shocking Police conduct from the Latvian mass media, have been guessed that media information had been heavily censored. Text from Critical Mass (English Wikipedia article)#Riga, Latvia.
On April 25th, 2008, several participants were arrested by the Riga, Latvia Police Department because of the event not being confirmed by the City Government and the participation of roller-skaters, which aren't allowed to ride on the road. Text from Critical Mass (English Wikipedia article)#Riga, Latvia.
At dusk on Friday, March 28, 2008 Honolulu Police tackled a young woman on a bike during a peaceful ride with other cyclists apparently in an attempt to discourage the cyclists from assembling and riding. The police officer's tackle caused her to fall from her bike and hit her head on the ground. She was subsequently hospitalized. No arrests were made. The incident occurred in front of the Honolulu Police main station on Beritania Street. In addition, Police issued citations to other cyclists before the officer tackled the woman. Text from Critical Mass (English Wikipedia article)#Honolulu, Hawaii.
Police repressions against Critical Mass in Vienna / (de):Polizeirepression gegen Critical Mass in Wien
(de): "Nachdem die CriticalMass nunmehr seit 16 Monaten friedlich und lustvoll durch Wien radelt und ein gutes Einvernehmen mit der immer wieder begleitenden Polizei herrscht, kommt es bei der CM im August völlig unverständlich zu einer Zerstörung dieses Verhältnisses durch die Polizei durch eine “Aktion-Scharf” gemäss STVO. [...]" See also .
"Around 40 cyclists started at the Heinrichplatz, during the ride the count increased up to 60 participiants. There was an incident at the Heinrich-Heine-Street where the police confiscated a bicycle (later gave it back) and got personal data of at least on person." See also Berlin, July 07.
Police Department officers grabbed and detained a peaceful Critical Mass rider on the Oakland, California ride, which stages at Frank Ogawa Plaza on the first Friday of each month. The rider was cited for violating a noise ordinance for having a bicycle sound system. Many Oakland riders were critical of the Oakland, California Police Department for exercising selective enforcement of the noise ordinance, in a city whose streets are frequently travelled by motorcyclists on Harley Davidson motorcycles with loud pipes and sound-systems of their own, and with numerous cars with loud sound-systems and exhaust whistles. In addition, Oakland Police Department motorcycles themselves were previously criticized by Oakland's District 3 City Councilmember Nancy Nadel for having loud, aftermarket "thunderpipes" installed on them. Text from Critical Mass (English Wikipedia article)#Oakland, California.
While the Chicago ride was heading to the Spindle in Berwyn, IL, an allegedly drunk driver plowed into the mass while attempting to flee the scene of a previous accident. 10 year old Will Healy, who was riding with his mother and sister, was directly hit by the car. Healy flew up onto the hood of the car, but was not severely injured. Several other bicyclists were injured or had their bicycles damaged while attempting to move out of the way of the car. The driver, Robert Rogers, 34, of Maywood, IL was quickly apprehended by police who were riding with and monitoring the group. See also .
Not a Critical Mass but one of the biggest cycle events worldwide ever took place. The Great Star Ride takes place every year in June as an official organized event (introduced by the ADFC). In 2007 up to 250.000 cyclists took part in this ride. The photo shows a part of the mass on a motorway.
According to a Bicycle Civil Liberties Union press release, a "motorist with his wife deliberately ran into the side of the monthly Berkeley Critical Mass bicycle demonstration"  and caused approximately $3,000 worth of damage to bicycles . However, the driver and two third-party witnesses told police that the bikers threw their bicycles under the vehicle. Subsequently, Critical Mass participants pounded on the hood and windows of the car and the windshield of the vehicle was shattered. See also .
San Francisco Mayor Gavin Newsom, in April 2007, requested that Critical Mass riders "police themselves." "It does the bicycle-advocacy community no good to have people that are aggressive and dispirit the entire movement,” Newsom said. “I would encourage the bicycle coalition to say, ‘Look, we don’t put up with this, enough is enough.’” See also .
Since the resurrection of the ride in April 2007, Sacramento Critical Mass  has been the subject of police scrutiny. The Sacramento Police Department dispatches several squad cars, motorcycles, and bicycle police to monitor the ride, despite the absence of any violent incident. The police follow the riders the whole time, and shout at them to stay all the way to the side of the road. Ironically, the police actually occupy a larger portion of on-street traffic than the riders and thus inhibit the flow of traffic far more than the riders do. In addition, the police use every justification to issue citations, which tends to keep the speed of the mass very slow. Citations in the past have included tickets for not having lights, for moving to far into the road way, failing to signal when turning left, etc. It is believed that the police's domineering presence and propensity for issuing as many tickets as possible, is an attempt to dissuade the riders from participating altogether.Text from the English Wikipedia article under the GFDL, see Critical_Mass_(English_Wikipedia_article)#Sacramento.2C_California.
On the evening of March 30, 2007 in San Francisco, toward the end of Critical Mass, Susan Ferrando of Redwood City, California claimed to have found herself in the middle of a group of bicyclists and tried to drive through them resulting in hundreds of bicyclists surrounding her minivan while her 11 and 13 year old daughters watched from inside. Ferrando denies striking a cyclist but claims they banged on the sides of her car, "keyed" the paint, and threw a bicycle through the rear window of the vehicle, causing $5,300 in damage. See also .
264 Critical Mass riders arrested at 2004 Republican National Convention protest activity
Between 5,000 and 6,000 participants took part in the Critical Mass bicycle ride. The monthly NYC Critical Mass ride promoted by the environmental group Time's Up! occurs on the last Friday of each month and prior to this ride had usually attracted about 1,500 riders. Police eventually blockaded roads and arrested 264 people in relation to that event. Most of them were charged with disorderly conduct and held in custody for 24 hours. This was the first time the NYPD had made any significant arrests of Critical Mass participants.
An incident occurred at the intersection of Elmwood Avenue and Summer Street in Buffalo, NY. Buffalo police stopped two cyclists for "failure to yield to an emergency vehicle." This led to an incident in which several people were allegedly attacked by police, and resulted in nine arrests of cyclists, and three convictions, including a well-known local journalist. The event attracted significant local media attention. See also . Text from: Critical_Mass (English Wikipedia article)#Buffalo, NY - "Critical Massacre".
Willie Brown, was asked by a journalist at a press conference how he planned to control Critical Mass. The Mayor was quoted in the press as threatening the riders with arrest, along with various belittling of bicyclists. The reaction and extensive press coverage grew into a confrontation at the July 25, 1997 ride. The local newspapers had published a route, although many riders were hesitant about—or flatly against—cooperation with it. On Friday the Mayor addressed the crowd at the Embarcadero (San Francisco) meeting place but was shouted down. The crowd of approximately 7,000 bicyclists quickly split into many parts, each being chased or monitored by police units, including helicopters. This resulted in extensive turmoil throughout the downtown area and many arrests and bicycle confiscations.
Return to the Scorcher, a film documentary by Ted White about cycling culture around the world was published. In the documentary George Bliss coined the name "critical mass" in his description of the traffic flow between bicycles and cars in China. The term "Critical Mass" was later adopted at the time of San Francisco's second ride (the first ride in SF was named "commute clot").