Some had "emission-free vehicle" painted on their backs.

"We're riding nude to see if this way they'll see us, so they don't run over any more of us," said student Alejandro Hernandez, standing naked before the ride.

"Being naked means we aren't invisible. The motorists don't respect us, they see us as a nuisance," he said.

To combat daily gridlock and chronic air pollution in the sprawling metropolis of about 20 million people, the capital's leftist government is promoting bicycle use and has begun building a network of cycling lanes.

But the lanes are still few and far between, so the city's growing number of cyclists must ride alongside old buses, trucks and stressed motorists.

Mexico City is one of the world's most polluted capitals, along with Beijing, blighted by its thin, high-altitude air and a ring of surrounding mountains that traps exhaust fumes from buses and factories on the city outskirts.

Authorities have worked to remove the worst-polluting vehicles from the road. But as the capital's population grows, the city gains up to 250,000 new cars each year. (Editing by Xavier Briand)