Residents reported a manhole overflowing with sewage near Centinela Avenue and Sepulveda Boulevard about 12:30 p.m. Wednesday, said Ron Charles, spokesman for the Los Angeles Public Works Department.
City crews dispatched to the scene determined the spill was caused by a blockage in a sewer main.
"The entire backup amount entered an adjacent storm drain, which discharges to the Sepulveda Channel, and ultimately, the Ballona Creek," Charles said in a statement.
Los Angeles and Culver City work crews diverted the flow of sewage, vacuumed up effluent streaming down a hillside and fixed the backup by about 5:30 p.m.
Dr. Jonathan Fielding, director of the Los Angeles County Department of Public Health, called it a "major spill," but did not know how much of the sewage had reached the ocean.
The two-mile stretch of beach south of Ballona Creek will remain closed to swimmers and surfers until at least 3 p.m. Saturday, he said.
In January 2006, 2 million gallons of raw sewage spilled from a Manhattan Beach pumping plant after an apparent power failure. Officials launched a massive cleanup after hundreds of thousands of gallons flowed onto the sand and into the ocean.
The last major spill happened in January in South Gate, when 210,000 gallons of sewage flowed through the L.A. River and emptied into Long Beach Harbor, county records show.
County health officials said they are closely monitoring and testing ocean water near the outlet of Ballona Creek. Beaches will reopen when they pass health tests for two consecutive days.