Los Angeles City Hall was evacuated on Friday during a tense two-hour standoff between the police and a motorist who drove his car onto the sidewalk outside the building and barricaded himself inside the vehicle with what appeared to be an oxygen tank next to him.

The bizarre incident, which began at about 4:10 p.m. Pacific time, stunned city workers who were finishing their workweek as the black sedan jumped the curb outside the building’s Spring Street entrance, coming to rest at the foot of the steps.

Zach Seidl, a spokesman for Mayor Karen Bass, said the car was nearly spewing smoke as Los Angeles police officers sought to make contact with the driver, who appeared to be suffering a mental health crisis. “As soon as I saw the car, it was clear something was wrong,” Mr. Seidl said.

Nearby, Los Angeles County sheriff’s deputies evacuated a downtown park, warning pedestrians of a potential bomb threat. A spokeswoman for the Los Angeles Fire Department said there were no reported injuries.

Mayor Bass, who was not at the scene, posted on X that City Hall had been evacuated “out of an abundance of caution.”

By 6 p.m., televised footage of the standoff showed officers taking the driver, an older man with long gray hair, into custody. A handicap placard hung from the car’s rearview mirror, and two handmade signs on the dashboard read, “Don’t want to hurt nobody” and “I need ur help D Trump US vet.”