As the light leaks out of the sky over downtown last Friday, mechanical gears squeak in the distance. Slowly, small groups begin to gather at Government Center. Before long, the groups swell into a crowd, then a swarm.

By 7:15 p.m., 4,000 people line NW First Street as if it's Tahrir Square. There are hipsters in tight jeans and tattoo sleeves, curvy Colombian women packed into spandex like sausages, and marching bands of Cubans in matching T-shirts. But instead of political placards, they've all brought bicycles. Instead of slogans, they offer sweat. Critical Mass is once again taking Miami's streets by storm.

Not everyone is happy about it, however. Motorists stranded in the sea of bicycles angrily blast their horns. Some try to push their way through the peloton. Meanwhile, half a dozen City of Miami cops lounge on their motorcycles at a gas station on the corner, not lifting a finger to rein in the riotous traffic.

Info Critical Mass on Collision Course With Cops and Motorists

"We were called here to make sure there were no fights, but that's it," says a hulking officer with salt-and-pepper hair and a stogie smoldering in his mouth. As he puffs on his cigar, the sea of cyclists begins moving west on its 12-mile route through Little Havana. The cop stays stoically seated.

"Look, they go right through the red light," he says. "We can't help them because what they are doing is illegal."

This ambivalence over Critical Mass pervades MPD. As the monthly bike movement has swelled in size over the past six years, Miami cops have steadfastly ignored it. While other cities including Miami Beach and Coral Gables have embraced the bikers, MPD continues to treat them as an annoyance, even arresting one rider last month for selling ice cream from his tricycle.

But there are real dangers to dismissing Critical Mass. Miami is one of the most dangerous cities in the country for cyclists. At least three bikers have been killed by cars in the past three years, yet many perpetrators of car-on-bike crime either aren't caught or face farcical penalties. Just last month, two Critical Mass participants were struck by a Mercedes-Benz belonging to a prominent local surgeon, yet MPD has yet to even interview the doctor. On Sunday, another car hit a cyclist on the MacArthur Causeway and sped off.

Mayor Tomás Regalado likes to boast that Miami is becoming a world-class cultural destination. But even as new museums go up downtown, the city is undermining one of its few real civic movements. It's time that Miami motorists, politicians, and cops embrace Critical Mass.

"Ideally, the police would embrace it and not clash with it," says Critical Mass Miami founder Rydel Deed, "because we're not going anywhere."

Before Critical Mass came along, this city's bike scene was limited to a couple of cruisers on Miami Beach and the occasional soccer dad pedaling through the suburbs. But in 2007, Deed imported the idea after a visit to Chicago, where he joined thousands of other cyclists on a short tour through the city. The monthly bike rides, which began in 1992 in San Francisco and spread worldwide, are equal parts cycling celebration and civic activism for bikers' rights.

Miami was one of the last major American cities to get on board. It was slow going at first. For several years, only 30 or so riders came out. But Deed and his fellow die-hards kept it going. By 2009, the rides had reached 100 members. They grew steadily until last September, when Dwyane Wade and Gabrielle Union tweeted photos of themselves at #CriticalMassMiami. LeBron James joined the movement two months later. Suddenly, Critical Mass was mainstream.

These days, the ride is in danger of getting too massive for Miami's comfort. Last Friday, the starting line stretched more than three blocks. The crowd was a mixture of Miami hipsters with sleek, ultralight fixed-gear bikes — or "fixies" — and moms sporting Walmart specials with ultrawide seats. Some people swigged beer from bottles. Others slurped water out of high-tech CamelBaks.

It's clear Critical Mass has become its own social scene. At Friday's ride, curvaceous young women wore see-through spandex with nothing underneath. One donned fuzzy devil horns, black tights, and a low-cut red halter-top. Many dudes, meanwhile, blared pop music from speakers mounted on their bikes. A heavyset woman in pink spandex balanced atop her bike like a bear on a unicycle. "Nowadays, some people buy bikes just to come to Critical Mass," Deed points out.

The mass began moving at 7:15, undulating westward along NW First Street. Like a marathon, there was some self-selection in the order, with those at the front racing away while those at the back of the peloton were forced to wheel along patiently. Near the back, one man with spiky hair immediately crashed right in front of the cops, who did nothing. The heavyset woman brought up the rear.

Spirits were high. But without the help of city officials or Miami Police, things quickly became disorganized. After the first 1,000 feet, a drawbridge rising above the Miami River chopped up what should have been a steady flow of cyclists into sporadic waves. Cars quickly surfed the spaces in between, honking their horns and occasionally swerving around cyclists. "Motherfuckers!" one SUV driver yelled.

"Yo, I'm riding a bike here, bro," shouted a scruffy dude in a yellow bandanna as a silver sedan tried to pass him in the left lane on Flagler. The cyclist slowed to taunt the car. "Where you going?"