(Adds San Francisco mayor, details; changes dateline from Oakland)

By Kimberly White

EMERYVILLE, Calif., April 29 (Reuters) - A stretch of vital highway for San Francisco Bay area commuters collapsed on Sunday after a fuel truck crashed and ignited dramatic flames more than 200 feet (60 metres) high, officials said.

San Francisco Mayor Gavin Newsom quickly ruled out sabotage or terrorism, but said the accident showed that the region's infrastructure remains dangerously vulnerable.

The flames burned through the upper deck of a highway interchange on the Oakland/Emeryville side leading to the double-decker Bay Bridge, which connects San Francisco to the heavily populated East Bay. A section of the highway collapsed in a 1989 earthquake.

The stretch of highway closed after Sunday's accident was used by nearly 500,000 vehicles every day, the California Highway Patrol said. Many workers use the highway to get to work in San Francisco from their homes in the East Bay.

"There is no evidence of foul play," Newsom told reporters at the California Democrats' annual convention in San Diego.

"It looks like an accident. The driver himself is in critical but stable condition and no one else seems to be hurt," he said. "But unquestionably this is going to be one of the most problematic commutes in recent memory."

Bay Bridge, around a half mile (0.8 km) from the closed interchange, remained open and was accessible from another highway. The overpass looked as though a slab above had melted onto the highway below near a large Ikea store.

Engineers were working to determine how much time and cost would be needed for repair. Newsom said he had spoken with the chief of staff of California Gov. Arnold Schwarzenegger in efforts to speed up repair of the vital highway.

The cause of the crash was not known, but the California Highway Patrol said there was no evidence the driver had been using drugs or alcohol.

Newsom said the accident was a wake-up call about vulnerability from natural and man-made disaster.

"These unfortunate events are opportunities to remind people that our infrastructure is not where it needs to be," he said.

"Obviously it's going to have an economic impact, a big one," he continued. "It is going to affect companies, it is going to affect job base and sales."

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