Hundreds of people were evacuated from the downtown high-rise after the man set himself on fire, The Los Angeles Times reported.
Witnesses said the man, in his 40s or 50s, came through the main entrance at around 3:30 p.m. carrying a plastic shopping bag, said Long Beach Fire Department spokesman Mike Duree. The man dropped the bag before setting himself on fire.
The bag was examined by bomb experts and found not to contain explosives, Duree said.
A civilian employee of the Long Beach Police Department who happened to be in the lobby at the time rushed the man and tackled him, and several other bystanders helped extinguish the flames. The police officer suffered minor burns on his hands.
Witnesses said the man smelled of gasoline, the newspaper reported.
The man was taken to St. Mary Medical Center where he was listed in critical condition, as he received treatment for first- and second-degree burns.
All the employees in the building were sent home, the Times said.
Police said no motive had been determined for the self-immolation.