Brazil leader slams WC protest 'campaign'

A subway strike snarling Brazil's biggest city before the World Cup has entered a third day, with President Dilma Rousseff denouncing a "systematic campaign" against her government and the tournament.

Some roads were congested in Sao Paulo on Saturday and commuters fumed in long bus lines in the latest labour upheaval to hit Brazil with just five days until the mega-city hosts the opening game.

Police, teachers and bus drivers have also staged strikes in other cities in recent months to demand better wages, while protesters angry at the World Cup's $US11 billion ($A11.90 billion) bill have staged demonstrations.

The subway standoff led to a clash Friday between picketing strikers and police inside a metro station, with authorities swinging truncheons and firing tear gas to disperse the protesters.

Officials are bracing for potential protests during the tournament, fearing a repeat of the violent demonstrations that marred the Confederations Cup last year.

Rousseff hit back at critics late Friday, charging that there was a movement to derail her Workers Party (PT) before October 5 general elections.

"Today there is a systematic campaign against the World Cup - or rather, it is not against the World Cup but rather a systematic campaign against us," she said in the southern city of Porto Alegre.

She said criticism of government spending on the tournament - notably stadia in some host cities with no football tradition - amounted to "disinformation".

The president did not say who was behind the campaign but she recalled a 2011 edition of conservative news magazine Veja that sardonically suggested the tournament's 12 stadium venues would not be ready for decades.

Rousseff, who was a leftist political prisoner during the 1964-1985 military dictatorship, said even then "we did not confuse the World Cup with politics".

Rousseff insists money spent on the tournament will leave a legacy of airports and transport infrastructure that will benefit Brazil for years to come.