Tens of thousands of East Bay bus riders could soon find their commutes upended after AC Transit workers gave notice on Monday that they were prepared to strike on Wednesday in the midst of ongoing contract negotiations.

In a letter delivered to the transit agency's Board of Directors as it met in Oakland, members of Amalgamated Transit Union Local 192, which represents 1,625 bus operators and mechanics, said they intended to go on strike 12:01 a.m. Wednesday unless their demands are met by then, said Clarence Johnson, an AC Transit spokesman.

The union has been operating on a contract that expired July 1, the same day BART unions' contracts expired.

A strike "would not be good," Johnson said. "Certainly it would mean we would not be able to operate service. We have about 181,000 passengers a day so that would mean that 181,000 people would be without their usual public transportation service."

AC Transit provides rides in parts of Alameda and Contra Costa counties, as well as in and out of downtown San Francisco. When BART seemed on the verge of shutting down again this week, before Gov. Jerry Brown intervened and called off the strike for at least seven days, AC Transit touted itself as a transportation alternative.

The sticking points concern wages and health care costs. Johnson said the transit agency last offered a 9 percent pay increase over three years, but union members are seeking an increase of 10.25 percent.

In addition, union members do not contribute to their health care premiums. AC Transit wants them to contribute 10 percent, Johnson said.

Margaret Hanlon-Gradie, a union spokeswoman, said workers have effectively been paying for health care by foregoing wage increases and ceding $30 million worth of concessions in their 2009 contract.

She also said drivers are seeking stronger safety measures, which could include increased security on routes.

"I think we're going to get there today or tomorrow, we hope," she said.