The problem was reported at about 5:25 a.m. on the line, which runs from Van Ness Avenue to the Embarcadero, Muni spokesman Paul Rose said.
There are mechanical issues with the cable, and the problem is affecting the entire line, Rose said.
Crews will be working throughout the day to fix the problem and hope to have service restored by Friday morning at the latest, he said.
Buses are carrying customers along the line in the meantime.
Dan McMenamin, Bay City News