OAKLAND — BART workers went on strike Friday and trains are shut down, setting the stage for the second Bay Area commute nightmare in three months.

Just after midnight, union leaders picked up picket signs and said they would not go back to work until they reach a contract agreement with management, stranding 200,000 people who ride BART roundtrip each day. Although workers had threatened strikes five times in the past week, this time they had finally reached their breaking point.

By late Thursday afternoon, talks had ended altogether and it was not clear when they would resume. A wild round of back-and-forth afternoon news conferences brought to a live TV audience an outpouring of emotion from sleep-deprived negotiators, and even the nation’s top mediator had given up.

“Unfortunately, yes — we are on strike as of midnight,” Antonette Bryant, president of the local Amalgamated Transit Union, said after the end of an epic bargaining session that began about 30 hours before and left negotiators wearing day-old clothes.

Both sides were inching closer on the main economic issues that had separated them for more than six months but were still about 4 percent apart on total wage increases. And unions said they were fed up after management tried to impose new work rules to limit overtime and other costs.

“It’s not management that asked for the strike — it’s the union,” BART General Manager Grace Crunican said, taking the microphone after Bryant went back inside the negotiating office in downtown Oakland. Some union members lingering around began to heckle her.

In the middle of it all, federal mediator George Cohen — who had been praised for nudging each side closer since joining the talks on Sunday — remarkably took to the podium to declare there was “nothing further we were able to do.”

“Unfortunately, regrettably, we were not able to bring them the result we all want to achieve: a voluntary collective bargaining agreement,” Cohen said. “Our mediation process has come to an end.”

Crunican said BART was continuing to propose its latest four-year offer. That includes a total 12 percent pay increase on top of union workers’ average gross pay of $76,500 — the highest among California transit agencies — changing pension contributions from zero to 4 percent, and bumping up monthly health care payments from $92 to $144.

Unions had agreed to the pension and health care offers but wanted a 15.9 percent increase in pay, BART said.

“I think we’ve offered the unions the best wage and benefits package in the country,” said BART board President Tom Radulovich, who said the agency was “very, very surprised” by the strike declaration. “There is no need for them to strike. We think it’s a terrible idea for the riders and the people of the Bay Area.”

The other big remaining issue is BART’s refusal to let a neutral arbitrator give the final ruling on various perks that workers want to keep but which management says are inefficient.

Among the work rules BART wants to change: Currently, union workers can call in sick, work four days and get paid overtime on the fifth day; employees can leave projects in the middle of a job to go work on something else; and employees can receive paper paycheck stubs instead of electronic notices.

The strike shutters the nation’s fifth-largest rail system again after a 4½-day walkout in July.

BART riders make up 5 percent of Bay Area commuters but adding even a small number of vehicles to already jam-packed freeways can significantly slow traffic. The Bay Area Council business group had estimated the strike would result in a $70 million daily hit to the local economy.

The average BART union worker loses about $290 in gross pay each weekday by walking off the job. After four years, the average union employee would make another $9,180 annually under management’s proposal and an additional $12,150 under the unions’ offer.

Transit agencies late Thursday began dusting off back-up service plans in preparation for a strike. That includes BART running up to 200 shuttle buses between the East Bay and San Francisco, while Caltrans was preparing to enforce all-day carpool hours on bridges and East Bay freeways.

A 60-day cooling-off period ordered by Gov. Jerry Brown twice averted threatened shutdowns in August but ended Oct. 10. Since then, in every day but two, the unions representing 2,300 blue-collar workers had threatened to strike, only to call it off each night sometime between 10 p.m. and 1 a.m.

But the heat at the long-simmering BART strike negotiations was turned way up Thursday when talks broke down.

“I’m sorry, I’m regretful,” said Roxanne Sanchez, president of the local Service Employees International Union. “I don’t know what to say to the public who has put such faith in the leadership of those who work at BART. The employer has been unwilling to reach an agreement or settle these disputes without a strike.”

Thursday marks 200 days since the opening of the negotiating period. Earlier in the day, BART’s chief negotiator, Thomas Hock, said “this should be the final stretch.”

“It’s a very difficult time,” Hock said. If they can’t reach a deal, “it’s not going to go down for a lack of trying.”

Staff writers Matthias Gafni, Gary Peterson and Doug Oakley contributed to this report. Contact Mike Rosenberg at 408-920-5705. Follow him at twitter.com/RosenbergMerc.