Muni officials said riders should expect crowding, delays and waits of up to an hour this evening. Some Muni vehicles may have to turn back mid-route to ease crowding, agency officials said.
"I want to apologize to our Muni customers and others who have been negatively affected by today's limited service," SFMTA director of transportation Ed Reiskin said in a statement.
Reiskin said that while Muni has an operations plan in place to help customers plan ahead, crowding and delays are still expected this evening.
Reiskin thanked Muni personnel "who were at their posts today and helped keep this city moving under very difficult circumstances."
Commuters and tourists scrambled to find ways to get around the city today as Muni operated at about a third of its normal capacity, with only about 200 of its roughly 600 vehicles in service, Muni spokesman Paul Rose said.
A Muni bus packed with riders made its way down Market Street near Civic Center around 10:40 a.m.
Meanwhile, a line of people at a Muni bus stop at Market and Fifth streets waited for an available bus.
One man, 28-year-old Duy Dao, said he was heading to work at the San Francisco Department of Public Works and had been at the bus stop for 45 minutes with no sign of his usual 9-San Bruno line bus.
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 today.
Many commuters, including San Francisco Supervisor Scott Wiener, took to Twitter this morning to vent their frustrations about the sickout.
"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.
Some Muni riders today have turned to BART, which is honoring Muni fares all day today between Daly City and downtown San Francisco, Muni officials said.
Representatives of the employee union, Transport Workers Union Local 250-A, have not returned multiple requests for comment on the sickout today.
The union has been in a contentious contract dispute with the SFMTA over the past two months and voted last week on a proposed agreement with the agency.
TWU Local 250-A members voted overwhelmingly to reject the proposal on Friday, according to results posted on the union's website.
Union president Eric Williams said in a statement on the site that it was an "unfair contract" and that the "city devalued our service as they proposed unreasonable take aways."
The proposed contract would give Muni workers an 11.45 percent raise over the next two years, bumping up operator pay to just under $32 per hour by July, according to Rose, the Muni spokesman.
However, the contract also proposes that workers contribute 7.5 percent to their pensions, an amount now covered by Muni.
Rose said the city pays 92.5 percent of transit operators' pensions, an amount he said "is in line with most other city workers."
The spokesman said Muni officials may not know until early Tuesday morning about whether another sickout is planned and said riders should know "some of the delays they saw today might occur tomorrow."
Riders are encouraged to check the agency's Twitter account at www.twitter.com/sfmta_muni or sign up for e-mail or text alerts at www.sfmta.com for updates.
More than 700,000 trips are made on Muni each day, according to Rose.