The rising floodwaters on the 10600 block of West Sunset Boulevard, which also toppled two large trees, shuttered the usually bustling street between Veteran and Hilgard avenues, according to the Los Angeles Fire Department. Stone Canyon Road between Sunset Boulevard and Bellagio Road was also closed.
An estimated reopening time was not immediately clear but drivers were asked to stay away from the area and find alternate routes.
No one needed to be rescued and no lives were put in danger by the flooding, which was reported around 1:30 a.m., officials said.
By 7 a.m., the Los Angeles Department of Water and Power turned off four valves to shut off the water main, which they said was about 30 inches wide.
UCLA staff stacked sandbags to protect Pauley Pavilion. The flooding didn't threatened student housing, said LAFD spokesperson Brian Humprey.
A university alert sent just before 5 a.m. said "all parts of campus are unaffected expect for Charles Young Drive North, parking lot 4, lot 7 and the Intramural Field."
That field sits above a parking lot where at least 10 cars were damaged by up to five feet of water, said LAFD Deputy Chief Armando Hogan.
Fire officials also said a residential property on the north end of Sunset Boulevard did take on some water.
The Los Angeles Department of Water and Power is working to identify the water main that broke and several valves would have to be shut off to stop the flow of water.
The water line break occurred not far from a 2014 break involving a 90-year-old water main that burst and sent between eight and ten million gallons of water onto the UCLA campus.
City News Service contributed to this report.