At least two people were injured near a stadium in the northeastern city of Fortaleza -- one of several sites hosting matches in the Confederations Cup and where elite police units have fanned out to protect the sites.
Some of the roughly 10,000 protesters hurled stones at security forces, who responded with tear gas and rubber bullets. One person suffered an eye injury and a second was taken away on a stretcher.
"Forward, forward," the demonstrators shouted as a cordon of police blocked access to the USD 240 million Castelao stadium. An indignant woman declared "this is a dictatorship" as police fired rubber bullets.
The clashes erupted only hours before Brazil faced Mexico in Fortaleza in their second Confederations Cup match following their 3-0 win against Japan in Brasilia Saturday.
The tournament, seen as a dry run for next year's World Cup, has been overshadowed by the biggest social unrest to hit Brazil in 20 years.
Protests initially sparked by a hike in bus fares in Sao Paulo quickly spiraled into nationwide marches against corruption, fueled by anger that -- in a country with a wide rich-poor divide -- billions of dollars were being spent on stadiums and far too little was earmarked for social programs.
Hundreds of thousands of people have taken to the streets across this continent-sized country to denounce the USD 15 billion being spent on this month's Confederations Cup and the 2014 World Cup.
The justice ministry said it had deployed a crack federal police unit in five places hosting Confederations Cup matches: the states of Rio de Janeiro, Bahia, Minas Gerais, Ceara as well as in the federal district of Brasilia.
The National Force, composed of police and firefighters from different states that are called up for duty on special occasions, is a "conciliatory, mediating" force, "not repressive," the ministry said.
In Fortaleza, a city of 3.5 million, 6,000 additional state police troopers were also deployed. "Brazil, we are going to wake up -- a professor is worth more than Neymar," the demonstrators in Fortaleza shouted, referring to a popular star of the national team.
Ahead of the match against Mexico, Neymar joined his Selecao teammates in backing the escalating social protests and chided President Dilma Rousseff's government for failing to deliver adequate social services.
"Saddened by all that is occurring in Brazil," Neymar said in a statement. "I always had faith that it would not be necessary to come to the point of having to take to the streets to demand better conditions for transport, health, education and security," he noted.
"While you watch television, I am changing the country. Football no, we want education," read one placard hoisted by a demonstrator in Fortaleza. The protesters also railed against the country's entire political class, which is widely seen as corrupt.
"We are protesting the use of public funds for the construction of stadiums, money that should be used for education," said 18-year-old Matheus Dantas, amid a sea of Brazilian flags. More protests were under way or set to take place later Wednesday, notably in Sao Paulo, Brasilia and Belo Horizonte.
On Tuesday, at least 50,000 people flooded the streets of Sao Paulo, Brazil's most populous city and the nation's industrial capital, to vent their anger at the country's politicians, including Rousseff.
Several hundred of them peeled off from the main march and set a car and a police stand on fire. Police fired rubber bullets and tear gas to disperse the crowd, but many returned and looted stores, taking off with jewelry, clothing and TV sets.
Early Wednesday, hundreds of protesters marched peacefully in suburban Sao Paulo. Faced with the widening protests, federal and state authorities made conciliatory gestures, with Rousseff vowing to listen to the voices of the country's angry youth and calling their demands legitimate.
And several cities, including Porto Alegre and Recife, announced a reduction in public transport fares. But in Sao Paulo, Mayor Fernando Haddad rejected a rollback of the fare hikes.
The mayor, who on Tuesday met in Sao Paulo with mentors Rousseff and her predecessor Luiz Inacio Lula da Silva, told a press conference that to do so would mean having to make deep cuts in other areas.
He warned that it would require subsidizing the transport sector to the tune of USD 1.35 billion until 2016. "We cannot take populist decisions," Haddad insisted.
In Rio, where 100,000 people rallied Monday in the country's biggest and most violent protest, Mayor Eduardo Paes conceded that local transport was of poor quality and expressed readiness to review the bus fares.
"It is the beginning of the tropical spring," leftist politician Givalnildo Manoel told AFP in Sao Paulo on Tuesday.