A group of about 150 strikers and 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.
But demonstrators regrouped with chants of "There won't be a Cup, there will be a strike!"
The protest grew to about 1,000 people, who marched toward the state transport secretariat waving red banners, banging drums and blowing whistles and vuvuzelas.
The subway strike has posed a major headache for commuters in Sao Paulo, a sprawling city of 20 million people, and threatens to disrupt transportation plans for Thursday's opening match between Brazil and Croatia.
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Police said they had detained 13 strikers for damaging a door and interfering with colleagues' work at a central subway station where officers used tear gas and truncheons to disperse picketers last Thursday.
The metropolitan transport authority for its part said it had fired around 60 strikers for "just cause."
Union president Altino Melo dos Prazeres condemned the move.
"That's unacceptable," he told AFP. "It's only going to aggravate the problem."
The union has reduced an initial demand for a 16.5-per cent wage hike to 12.2 per cent, but the government is offering only 8.7 per cent.
A labour court yesterday ruled 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.
The subway has been partially operating, but trains were not arriving at the stadium.
Prazeres said he was confident the strikers had the upper hand.