The actor, 26, was punched in the mouth, TMZ reports, while officer Wendy Reyes, a spokesperson for the LAPD, confirms to PEOPLE that the alleged victims claim they ran out of gas, which is why they were in the area.
No one was arrested in the incident, as police believe the fighting was mutual while Efron waited for a tow truck.
The star of That Awkward Moment was "obviously intoxicated," a source told TMZ. His rep has not yet commented.
Hours later Efron was spotted in high spirits Sunday night at the Hollywood Improv's Laffest show.
Noshing on nachos and chicken wings while sipping on green tea, Efron took in comedy from the nine different up-and-coming acts, alongside a male and female pal, an onlooker tells PEOPLE.
"The group laughed a lot," the source says. "Zac was a sweetheart to staff. He appeared very relaxed. He and his two friends kept to themselves. He appeared sober and nothing at all seemed out of the ordinary. He did not look injured or different by any means."
Sources close to the actor, who told fans at the SXSW Film Festival premiere of his new movie Neighbors on March 8 that he feels "blessed", say that Efron has worked hard at staying sober since a stint in rehab last spring.
"He has temptations," an industry source who has spent time with Efron told PEOPLE.
Efron himself has told friends that recovery is "not easy." When he broke his jaw this past November and had to have his mouth wired shut, insisting he had slipped on a puddle at home, some worried he could be having a post-rehab relapse. Efron vehemently denied rumors that partying caused the Nov. 11 injury.
"He cut most everyone out of his life who were part of the party scene," a source in his circle told PEOPLE.
Friends say Efron has been focused on reading scripts, working out, going to 12-step meetings and honing a new support system.
"He's made a remarkable effort to straighten up and concentrate on staying that way," an Efron pal said. "He wants to be healthy."
While at SXSW, a low-key Efron promoted Neighbors at a Facebook Live event alongside co-stars Seth Rogen, Dave Franco and Christopher Mintz-Plasse, and mingled with the crowd at the film's fraternity-themed afterparty, but insisted at a later event that his favorite beer was nonalcoholic O'Doul's.
"It was an interesting year," he said in January. "The best part of it was being able to reflect upon that experience and realize how much I have learned about myself – and the kind of man I want to be."
Additional reporting by ELAINE ARADILLAS and DAHVI SHIRA