With 100 of precincts reporting from Tuesday's election, initial results indicated Garcetti captured 93,978 votes, or 32.9 percent while Gruel received 83,308 votes, or 29.2 percent, the Los Angeles Times reported. In a distant third was attorney Kevin James, with 46,684 votes, or 16.4 percent.
The two top finishers in Tuesday's election will face off in a May 21 runoff.
Sixteen percent of the city's 1.8 million registered voters cast ballots in the election, the Times said.
"We have the most votes tonight, and it looks like we are heading to a runoff," Garcetti told supporters. "I'm ready to work as hard as it takes; I'm ready to get up as early as it takes. Tomorrow we're going to get up, we're going to get to work, and we're going to win this campaign."
Greuel told her supporters late Tuesday, "Although not all the votes are in, it sure looks good. ... We're 11 weeks from making history, electing the first woman mayor. And, of course, the first mom."
The vote for a measure to would have added a half-penny to the city's sales tax seemed to be headed for defeat, the Times said. Proposition A, would have raised the sales taxes in Los Angeles to 9.5 percent, among the highest rates in the state, and raised $200 million a year for the city treasury.