The sixth in a series of protests against a hike in public transit fares in Brazil's largest city ended in violence early Wednesday.

Riot police, who were criticized for their heavy-handed repression of demonstrations last week, stayed on the sidelines for hours as some 50,000 people took to the streets of Sao Paulo late Tuesday.

But as the protest wore on past midnight and isolated incidents of looting popped up, the cops turned aggressive.

"If you don't get out of here, we're going to shoot you," one police officer said to three middle-aged women seeking shelter from the disturbances.

"I'm still shaking now," one of the women said later.

Authorities attributed the looting to vagrants who decided to take advantage of the chaos to grab merchandise from shops.

The protest over the transit hike, organized by the MPL movement, was largely peaceful, though a small group of militants tried to force their way into city hall, shattering windows.

Police fired tear gas and mounted several charges into a crowd of around 5,000 people gathered in a square next to city hall.

Some among the protesters then set fire to a police sentry-box and to a truck belonging to TV Record, an outlet that has been critical of the MPL demonstrations.

After starting last week in Sao Paulo and Rio de Janeiro, the protests over the transit hikes spread to dozens of cities and some 250,000 people nationwide took part in demonstrations on Monday.

Besides the issue of transit fares, many Brazilians are unhappy about the amount of public money spent on hosting sports extravaganzas such as the soccer Confederations Cup - now in progress - and the 2014 World Cup.

"There are no medicines in the hospitals, there are no materials in the schools. They invest only in that World Cup," 17-year-old Ana Neimeier said during Tuesday's march in Sao Paulo.

At dawn Wednesday, hundreds of protesters blocked two main highways in greater Sao Paulo with piles of burning tires. EFE