SAN FRANCISCO — A fire near a Bay Area Rapid Transit station shut down train service between San Francisco and Oakland today, snarling the morning commute as thousands of people scrambled to find other ways to get around.

In lines stretching for blocks, commuters waited to catch a bus or ferry boat into San Francisco. Traffic on the San Francisco-Oakland Bay Bridge was heavier than usual as many drivers waited for more than two hours to cross the bridge, the California Highway Patrol said.

BART officials expected to restore full service between San Francisco and the East Bay by 4 p.m. About 400,000 people take BART trains on a weekday, the transit agency said, but more commuters than usual were expected due to the opening round of the U.S. Open golf tournament and a San Francisco Giants afternoon baseball game.

“This is a major inconvenience to tens of thousands of people, and we’re working as hard as we can to get us back in service safely and as quickly as we can,” BART spokesman Jim Allison told KGO-TV.

Brian Long of Oakland waited in a ferry line, hoping to make it to work in San Francisco before noon.

The accountant said he needed to be in the office for an important reason: He was waiting to find out if he won a free trip to Hawaii in an in-office weight-loss competition. Long said he lost 66 pounds in four months and was sure he had won, “but I have to be there in person to claim the prize.”

Karen Hernandes from Concord said she decided to take the ferry for the first time after taking BART to downtown Oakland and passing a bus line that snaked for blocks.

“Someone told me this was the better option,” said Hernandes, an executive assistant at a San Francisco real estate company. She eventually made it to work — nearly four hours after leaving home.

Ernest Sanchez, a transportation services manager with the San Francisco Bay Ferry, said the service added extra boats throughout the day. He said the service experienced five times its normal daily traffic, leading to a temporary crash of its website.

The shutdown also fouled the commute to the East Bay. Kyle Neesan was among those stopped at a downtown station in San Francisco, where trains were backed up.

“It’s kind of annoying,” said Neesan, who takes BART to his job at a Sports Authority in Concord. “I’m not going to wait on BART because that could take forever. I guess I’ll just try to walk over to the bus.”

Rae Lyn Burke said the shutdown would keep her from making a morning meeting in Lafayette.

“I’m trying to figure out how to get there, but I don’t think it’s going to work,” she said, as she searched her iPhone for alternatives in the crowded Powell Street station.

The fire broke out around 2:15 a.m. at a retirement home that was under construction near the West Oakland BART station, BART officials said. It damaged electrical equipment, but train tracks and the elevated concrete structure supporting them were not harmed, Allison said.

BART officials were concerned about power poles that were in danger of falling on the tracks.

The fire jumped to several other structures and also melted parked cars. Its cause is under investigation, but authorities consider the blaze suspicious.

Oakland Fire Battalion Chief Robert Lipp told KGO-TV the fire grew rapidly, and there were reports of people in the area who are not normally there.

BART was running trains throughout the East Bay and between San Francisco and the Peninsula.