BART is closing down a key portion of its system in Oakland this weekend as part of a major rail replacement project -- a shutdown that will disrupt service between the East Bay and San Francisco.

There will be no BART service between West Oakland and 19th Street stations or between West Oakland and Lake Merritt stations. The work will start at 1:30 a.m. Saturday and end just before the system opens for the Monday morning commute.

The agency will provide a free shuttle service using AC Transit buses between 19th Street and West Oakland.

BART spokeswoman Anna Duckworth said Thursday the work will be on the several lines that converge at West Oakland.

“It’s a really critical part of our system,” Duckworth said. “We’re going to be replacing welded rail as well as replacing restraining rail, which acts as a guardrail for our wheels.”