OAKLAND -- Marathon negotiations between the two largest unions representing BART employees and management concluded shortly after 11:30 p.m. Saturday, with encouraging signs that the two sides were making progress to avert a Monday strike that could leave tens of thousands of commuters in paralyzing gridlock.

But it remained unclear how close the two sides were to hammering out an agreement after nearly 14 hours of talks at the Caltrans building.

Lead BART negotiator Tom Hock said shortly after 11:30 p.m. that both sides had decided to adjourn for the night because it was getting late and they didn't want to make any mistakes with so much at stake. He said talks would resume Sunday at a time to be determined with a state mediator.

"We've all been working very hard to get it done," Hock said.

He declined to characterize how much progress had been made but said he was encouraged that the talks had gone so late.

A union representative said earlier in the evening that negotiators understood they are racing the clock before Sunday night's strike deadline.

"We are working on it," said Antonette Bryant, president of Local 1555 of the Amalgamated Transit Union, shortly before 7 p.m. "The unions understand the gravity of the situation. We don't have a lot of time."

After talks concluded Saturday, Bryant said her union would resume negotiations at 11 a.m. Sunday. She said Saturday's talks ended when they did because BART negotiators need guidance from higher-ranking management on how to respond to two union proposals.

She wouldn't go into details about the proposals, citing a gag order from the state mediator.

Bryant said earlier the two sides were discussing the difficult issues of pay and benefits in a session that started at 10 a.m. and continued with a working dinner session.

Although the situation was tense as the strike deadline loomed, there was also some levity Saturday. Hock, outside for an afternoon beverage break, took a reporter's microphone and pretended to interview the media.

On Thursday, BART unions gave 72 hours notice of a strike, which would begin Monday morning. The strike would be a resumption of a work stoppage that lasted for four and a half days during the week of July 1 before both sides agreed to a 30-day cooling-off period. The cooling-off period expires at 11:59 p.m. on Sunday.

BART met with negotiators from ATU, the second-largest union representing BART workers, until 8 p.m. Friday, BART spokesman Rick Rice said. Management met with representatives from SEIU until 9:30 p.m. Friday, Rice said.

Also Friday, state Assemblyman Mark DeSaulnier, D-Concord, released a statement urging the transit agency and its workers to reach a settlement.

"The top priority for BART and its labor unions must be to continue to engage in serious negotiations and avert a possible strike," DeSaulnier said. "I know that it is no easy task to find a solution that is fair to workers, is mindful of budgetary constraints, and ensures BART delivers top-notch safety for both riders and workers. BART is a major component of the Bay Area economy, and another prolonged strike could be devastating for the commuters who rely upon BART every day."

After the four-year contract between BART and its two largest labor unions expired at the end of June, the agency's 2,300 blue-collar union workers went on strike, shutting down the nation's fifth-largest rail line for 4½ days. A 30-day agreement to get the trains running expires at 11:59 p.m. on Sunday.

In the month since the first strike ended, neither side has reported significant progress on the three main issues that continue to divide them: pay, medical benefits and pensions. That led unions on Thursday to file an official 72-hour notice to strike Monday morning if no progress is made before then.

BART union workers have not received a raise in four years. They make an average of $76,500 in gross pay annually -- the best among California transit agencies -- contribute nothing toward their pensions and $92 monthly for health care.

The most recent proposals released publicly earlier in the week from BART included a four-year raise totaling 8 percent, while unions countered with a three-year pay increase of more than 20 percent. They were several percentage points apart, as well, on raising contributions to pensions and medical benefits.

If a strike happens, officials predict a 10 percent increase in traffic over the last strike, which happened during a holiday week and still produced gridlocked roads and packed buses, trains and ferries.

Transit agencies around the Bay Area would slightly beef up service and BART would run a limited bus service from the East Bay into San Francisco. Casual carpool lots would be expanded, the carpool lanes on Interstate 80 would be enforced all day and bridge toll booths would be fully staffed.

Staff writer Mike Rosenberg contributed to his report. Contact him at 408-920-5705. Follow him at twitter.com/RosenbergMerc. Contact Gary Peterson at 925-952-5053. Follow him at twitter.com/garyscribe.