SAO PAULO (AFP) - Police detained hundreds across Brazil overnight during fresh clashes with youths protesting an increase in mass transit fares on the eve of the Confederations Cup.

The worst violence was in Sao Paulo, the country's economic capital and most populous city, where 232 people were taken into custody downtown, a police spokesman said on Friday. Most were later released.

Twelve police were hurt during the unrest, he added, while local news reports said the total number of people injured topped 100.

The Sao Paulo protests, which began last week, have been spearheaded by the Free Pass Movement, which opposes a recent hike in bus, metro and train ticket prices from US$1.50 (S$1.88) to US$1.60 and wants free rides for students.

The group has called for another mass demonstration on Monday.

In the capital Brasilia, some 200 homeless activists burned tires and formed human chains on Friday to block access to Mane Garrincha, one of the six host stadiums for the Confederations Cup, police said.

Brazil faces Japan there on Saturday in the opening game of the two-week football tournament, which is seen as a dry run for the World Cup.

A police spokesman said the protest was organised by the Homeless Workers Movement (MTST), which is campaigning to reduce a housing shortage by squatting in abandoned government buildings.

In Rio, meanwhile, another of the six Confederations Cup host cities, around 20 people were detained late on Thursday following a demonstration by 2,000 people, mainly students, to demand lower bus fares.

Smaller protests over the same issue were also held in Porto Alegre, where 23 people were detained and seven hurt.

In the central city of Goiania, the local transport company suspended the fare hike in response to protests.

In Sao Paulo, home to 11 million people, police stepped up efforts overnight to prevent a repetition of widespread acts of vandalism and lawlessness that accompanied earlier protests.

But small groups of vandals managed to set fire to garbage in the streets, while others smashed store windows or spray painted buses.

Under orders to prevent the demonstrators from reaching the main Paulista Avenue, riot police fired tear gas and rubber bullets and used their batons, injuring several people, including reporters.

Reporters Without Borders has slammed the military police's treatment of journalists covering the protests.

Amnesty International's Brazilian branch also weighed in, slamming "a radicalisation of repression and the jailing of journalists and demonstrators" but also condemning acts of violence by both sides and urging dialogue.

"It is fundamental that the right to protest and the staging of peaceful demonstrations be ensured," the rights group said.

Sao Paulo Mayor Fernando Haddad conceded that, based on television footage of the violence, the police may have used excessive force.

But he and Sao Paulo State Governor Geraldo Alckmin ruled out a rollback of the fare hikes, saying they were as low as possible and "below inflation". Mr Alckmin defended the police conduct, dismissed the protests "as political", and condemned the widespread destruction of public property by a "small minority".

Thursday's Sao Paulo demonstration began with an estimated 5,000 youths - many of them students waving red flags of the Trotskyist Unified Socialist Workers' Party and chanting leftist slogans - massed outside the Baroque-style Municipal Theater near City Hall.

Some banners read "We will not tolerate being exploited" or "Our rights have a price".