Two people have been shot and killed on the University of California, Los Angeles (UCLA) campus, according to a Los Angeles Police Department (LAPD) spokesman.
The campus on the city's west side was locked down and officers with the LAPD and the university's police force were running across the campus, some with weapons drawn.
At one point, a team in tactical gear stormed into a building and more than a dozen police cars gathered near Boelter Hall, an engineering building.
People emerged from buildings with their hands raised or behind their heads. Police told people to stay away from the campus, in a bustling part of the city.
Two people were confirmed dead, LAPD officer Aareon Jefferson said.
Campus media relations officer Rebecca Kendall said she did not know the whereabouts of the shooter.
The shooting occurred the week before final exams at the school, the flagship of the University of California system with about 43,000 students.
Many students posted to social media, many to let friends and family know they were safe.
Some described frantic evacuation scenes and a large police response, others wrote of sitting in darkened rooms after barricading doors.
The focus of police activity was a cluster of engineering buildings near the centre of a campus that occupies 419 acres in Los Angeles.
Bioengineering professor Denise Aberle said she could see "a lot of police activity with innumerable cars" and a police helicopter hovering over the engineering building.
"Police keep coming," she said.