SAN FRANCISCO (AFP) - Tens of thousands of San Francisco commuters face possible prolonged travel disruption after the first strike by the city's mass transit system for 16 years started on Monday.

The Bay Area Rapid Transit (Bart), which carries 400,000 people a day in and around the West Coast city, closed down from midnight after wage negotiations between unions and bosses fell apart.

Train operators, mechanics, station agents and maintenance workers want a five-per cent pay increase, and are fighting efforts to force them to contribute to pensions and health insurance.

Freeways were snarled and buses and ferries that were working packed as commuters and tourists sought alternative means to get to their destination.

"We are sorry peoples' lives have been disrupted by the union strike... Unfortunately we do not have any further information on when negotiations will resume or how long the strike may last," said Bart spokesperson Rick Rice.

"This strike is not necessary and we call on union leaders to end it and join us at the table so the Bay Area can get moving again." Union spokesman Josie Mooney said: "A strike is always the last resort, and we have done everything in our power to avoid it.

"We are disappointed that Bart's failure to bargain honestly and fairly means that hundreds of thousands of Bay Area commuters have to suffer," added Ms Mooney, of the SEIU Local 1021 union.