Story highlights Thousands of demonstrators protest hike in bus and metro fares

Police in riot gear use tear gas on the protesters

Some demonstrators damage a metro station and burn buses

The protests are led by a group calling for free or cheaper transportation fares

Police arrested 25 demonstrators after clashes Tuesday in the streets of Brazil's largest city, protest organizers said.

Under heavy rain, thousands of demonstrators took to the streets to protest planned increases in public transportation fares in Sao Paulo. The protests were led by a group known as Free Fare, which advocates free or cheaper public transportation.

Police in riot gear clashed with protesters and set off tear gas, images from CNN affiliate Band News showed.

Some demonstrators turned violent, damaging a metro station and setting fire to several public buses in one of Sao Paulo's busiest streets, according to Band News footage.

This is the third time the group, mostly composed of young protesters, has taken to the streets. Tuesday's demonstration was the most violent one so far.

In a post on Free Fare's official blog, one of the group's members, Daniel Guimaraes Terschitsch, accused police of starting the violence.

Police officials didn't immediately respond to a request for comment.

Transportation authorities in Sao Paulo recently increased bus and metro fares from 3 reais (about $1.40) to 3.20 reais (about $1.50).