RIO DE JANEIRO (AFP) - Thousands of Brazilians threatened fresh protests Tuesday over the huge costs of preparations for the World Cup, after the biggest demonstrations in 20 years shook Dilma Rousseff's government.

On Monday more than 250,000 people marched in major cities, railing against the government's investment of billion of dollars in major sporting events while, they allege, spending on education and health suffers.

The protests, held as the country staged football's Confederations Cup, a dry run for the World Cup, were largely peaceful, although clashes with police and acts of vandalism were reported in Rio, Porto Alegre and Maceio.

In Rio, where 100,000 marched, some tried to storm the state legislative assembly, set fire to a car and ransacked shops. Twenty police were hurt, along with several demonstrators.

Police used tear gas and rubber bullets to disperse rowdy crowds and vandals in Belo Horizonte, where protesters tried to march on the Mineirao stadium, venue for a Nigeria-Tahiti Confederations Cup game.

Monday's demonstrations were the biggest in Brazil since those against corruption in 1992 under the rule of Fernando Collor de Mello, who was subsequently impeached.

"The government is worried," admitted President Rousseff's chief of staff Gilberto Carvalho, urging the oppposition not to get involved. "No one from either side should try to take advantage of the situation."

New demonstrations were scheduled for Tuesday in Sao Paulo and for Thursday in several other cities, including Rio, one of the hosts of the Confederations Cup.

On Thursday, any unrest could affect the match between Spain and Tahiti in Rio's Maracana stadium and between Nigeria and Uruguay in Salvador.

The spark from the current wave of unrest was a nationwide hike in public transport fares but this quickly broadened to resentment over the billions of dollars the government is investing for the Confederations Cup, the World Cup and the 2016 Rio summer Olympics.