A Brazilian court has ruled that a subway strike that has caused traffic mayhem in Sao Paulo, raising fears of transport chaos at this week's World Cup kick-off, is illegal.

The judges ordered the subway worker's union on Sunday to pay a $222,000 fine for every day that the strike continues, up from the $45,000 they had to pay for the strike's first four days.

The union said it voted in favour of continuing the strike that was called "abusive" by the court.

"Clearly, the court ruling was a big blow to the unions," said Al Jazeera's Gabriel Elizondo, reporting from Sao Paulo.

"FIFA and the Brazilian government are watching this very closely as the majority of the tourists and football fans in Sao Paulo will be using the metro or train to go to the opening match this Thursday."

The five-line subway system has been partially operating, but trains were not arriving at the Corinthians Arena that will host the opening game.

Worker demands

Union workers have reduced an initial demand for a 16.5-percent wage hike to 12.2 percent, but employers are offering 8.7 percent.

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

Authorities are desperate to resolve the latest labour dispute to avoid another embarrassing incident in a World Cup hit by delays, cost overruns and protests, even before it has started.

The 61,600-capacity Corinthians Arena has itself been plagued by delays, and construction workers were racing against the clock over the weekend to finish it before the opening whistle.