The officer was trying to conduct traffic control around 7:30 p.m. near the intersection of Martin Luther King Jr. Boulevard and Woodlawn Avenue when he was hit, the department said.
The driver who was involved in the crash was said to be cooperating with authorities. The officer, who was not immediately identified, was taken to the hospital with serious injuries.
Police initially said he was stable, but later said he was in critical condition. Additional details about his injuries were not disclosed.
LAPD tweeted late Friday night that the officer "is receiving the best treatment possible and he is surrounded by experts and the love of his colleagues and family."