Police in Sao Paulo fired tear gas Monday to disperse protesters supporting a subway strike that has unleashed transport chaos three days before the Brazilian mega-city hosts the World Cup kick-off.

A group of about 150 protesters set fire to piles of garbage to block a central avenue in the Brazilian business hub, prompting some two dozen riot 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 between Brazil and Croatia.

Demonstrators from various activist groups shouted "No to repression!" before police broke them up.

But as three police helicopters hovered overhead, the protest regrouped and grew to about 300 people who marched up the same street chanting "There won't be a Cup, there will be a strike!"

Police also used a stun grenade against a separate group of about 70 picketing strikers, said union president Altino Melo dos Prazeres.

Police said they had detained 13 of the workers for damaging a door and interfering with colleagues' work at the station, where officers used tear gas and truncheons to disperse strikers last Thursday.

The union has reduced an initial demand for a 16.5-percent wage hike to 12.2 percent, but the government is offering only 8.7 percent.

A labor court ruled Sunday that the strike was illegal and imposed a $222,000 fine for every day it continues, but the union voted to press on with the walkout.

But Prazeres said he was confident the strikers had the upper hand.

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

Rising inflation and a sluggish economy have tarnished the World Cup glow in Brazil, fueling the anger of strikers and protesters who say the $11-billion budget would have been better spent on education, health and transport.

The protests come as World Cup teams continue arriving in Brazil, with reigning champions Spain touching down Sunday night.

France, Cameroon, Argentina, Uruguay, Ecuador, Costa Rica, Honduras and the United States are all due to arrive Monday -- the latter three in Sao Paulo.

After that all but five of the tournament's 32 teams will be on the ground.

UN Secretary General Ban Ki-moon and 11 heads of state and government from around the world are expected at Sao Paulo's Corinthians Arena for the opening ceremony and match.

Around a billion people worldwide are expected to watch on TV.

But the chaos gripping Sao Paulo -- the latest in a wave of strikes and protests around the tournament -- has distracted the usually passionate footballing nation from the World Cup buildup.

Some 4.5 million people use the city's five-line subway system every day and it is the main transport link to Corinthians Arena.

The subway has been partially operating, but trains were not arriving at the stadium.

Authorities are keen to resolve the latest labor dispute and avoid further embarrassment in a World Cup hit by delays and cost overruns even before it has started.

Corinthians Arena has become a symbol of the problems besetting the tournament.

At the weekend workers were racing to finish the 61,600-capacity stadium, which has been chronically behind schedule and over-budget.

Work on the 12 host stadiums has also been overshadowed by accidents that have killed eight workers. Three of the deaths were at Corinthians.

President Dilma Rousseff, who is seeking reelection in October, insists the money spent on the tournament will leave a legacy of modernized airports and transport infrastructure that will benefit Brazil for years.

But many of the promised projects have been shelved, adding to protesters' anger.

A year ago during the Confederations Cup, a World Cup dress rehearsal, more than a million people flooded the streets, some trashing property and clashing with police.

Recent protests have been smaller, but activists are vowing to revive last year's "Tropical Spring" during the World Cup. (AFP)