A sickout by Muni workers over a labor contract caused major service disruptions Monday morning, with transit management apologizing for the delays.
In a post on Twitter, officials with the transit agency warned riders to "expect major delays on Muni today. Working to balance service the best we can across the city. Apologies for this inconvenience."
Muni officials also warned commuters to expect the same level of disruption to the service during Monday's evening commute.
Paul Rose, a Muni spokesman, said 400 of the 600 Muni vehicles that are normally on the streets were not in service. Some riders reported waiting more than an hour for a bus to show up.
"I can't confirm it's a sick out, other than to say more operators called in sick and we're trying the best we can to provide service across the city," Rose said.
Duy Dao, said he was heading to work at San Francisco's Department of Public Works and had been at the bus stop for 45 at Market and Fifth Streets, with no sign of his usual 9-San Bruno line bus.
"I'm still waiting," Dao said.
Italian tourist Giada Acquistapace, 30, said she'd been waiting at the stop for about 25 minutes and was pondering alternative modes of transportation to get to the Golden Gate Bridge.
San Francisco Supervisor Scott Wiener tweeted: "Walking from Castro to City Hall due to Muni driver illegal sickout. Thank you to the Muni drivers who actually showed up to do their jobs," Wiener wrote, referring to a law passed by voters in 2010 that prohibits Muni workers from striking.
And Orinda City Councilman Steve Glazer, a Democrat who is running for state Assembly on a no-strike platform for transit workers, issued a statement saying that public workers deserve a fair wage. But these "job actions," he said, are "counterproductive and "create enourmous animosity and anger by riders and taxpayers."
BART honored Muni fare in San Francisco and Daly City, Muni officials said. Several taxi companies reported being inundated with calls.
The sickout comes amid displeasure over a proposed labor agreement that was the subject of a vote Friday by Muni operators.
The outcome of the vote wasn't immediately known, but officials with Transport Workers Union Local 250-A had voiced concerns about the Municipal Transportation Agency's offer, according to the San Francisco Chronicle.
Under the proposal, the agency says, Muni workers would get 11.25 percent raises over two years, but they would pick up a 7.5 percent pension payment now paid by the MTA.
The contract would push operator pay to about $32 an hour July 1, making Muni drivers the second-highest-paid transit workers in the country, according to the MTA.
Muni workers, like all San Francisco city employees, are prohibited from striking. Under a law approved by voters in 2010, if the union rejects the contract, the two sides go before an arbitrator who cannot rule against Muni management's proposals unless the union proves its interests outweigh "the public interest in efficient and reliable transit."
Union officials have complained that the standard is too high. In a statement on the union's website last week, Local 250-A president Eric Williams called the arbitration procedure "a lopsided and unfair process. The membership must take a stand, which will be communicated through your vote."
Union officials said, however, that they were not recommending a "no" vote.
The union representing the operators - Transport Workers Union Local 250-A - and Muni have been in contract talks since the start of the year.
NBC Bay Area's Kris Sanchez, The Associated Press and Bay City News contributed to this report.