Police in Sao Paulo have fired tear gas to disperse around 150 protesters supporting a subway workers' strike.

The action has unleashed transport chaos in the Brazilian city three days before it hosts the World Cup kick-off.

The protesters set fire to piles of rubbish to block a central street in the Brazilian business hub, prompting police to fire stun grenades and then tear gas to disperse them.

The demonstrators were backing a five-day-old strike by subway workers that has posed a major headache for the sprawling city of 20 million people ahead of Thursday's opening match.

Police also used a stun grenade to break up a separate demonstration by about 70 striking workers.

They had gone into a central metro station to try to convince supervisors to join the strike, union president Altino Melo dos Prazeres said.

Mr Prazeres said he was confident the strikers had the upper hand in their negotiations with the government for a pay increase.

"I don't believe the government wants to thwart this cup," he told AFP news agency.

Sao Paulo's metro workers voted to stay off work for a fifth day today even after a court declared the strike illegal.

Another vote on the strike was scheduled for today.

A court yesterday set a 500,000 reais penalty (€163,000) for each day they stay off work from today.

It also declared the strike illegal, paving the way for state-owned Companhia do Metropolitano de Sao Paulo to lay off striking workers.

Metro workers demand a 12% pay rise, but Metro has offered 8.7%.

With major subway lines closed since Thursday, commuting in Brazil's largest city has been chaotic.

The strike snagged several FIFA officials in over two hours of traffic as they arrived for a conference ahead of the World Cup last week.

On Friday, police used tear gas to break up a demonstration blocking access to one metro station.

Frustration with broken promises and the ballooning cost of new World Cup stadiums contributed to widespread protests that drew over a million Brazilians into the streets during a warm-up tournament last year.

This year, the largest demonstrations so far have been from homeless groups and striking workers using the backdrop of the World Cup to press their causes.