Emergency services found patrons “pouring out of the location screaming” when they arrived, according to Captain Andrew Meyer.
Then, police declared that 10 people had passed away.
At least 10 additional people have suffered injuries and are being treated in nearby hospitals for conditions ranging from critical to stable.
It was too early to say whether the incident was being treated as a hate crime, according to investigators, who also stated that there was no obvious motive.
They declared the cancellation of the Lunar New Year festival activities scheduled for Sunday.
Captain Meyer added that Alhambra, a neighborhood north of Monterey Park, had experienced an incident, but he did not go into further detail.
Investigators are on the scene looking for any connections between the two incidents, he said.