LOS ANGELES (AP) _ A car struck and injured four children playing on a sidewalk Friday night, and bystanders attacked the driver, authorities said.

A 6-year-old boy and his 3-year-old sister were critically injured in the accident in the Highland Park area of Los Angeles, said Fire Department spokesman Brian Humphrey.

The boy did not have a pulse, and it was not immediately known if firefighters were successful in resuscitating him, he said.

A 5-year-old girl with serious head injuries and a 4-year-old girl with minor injures were also taken to the hospital.

Firefighters arrived to find a large group of bystanders involved in an altercation with the driver who witnesses said was speeding, Humphrey said.

``A large and unruly crowd had gathered and several were involved in fisticuffs with the driver,'' he said.

The driver did not require medical attention, he said.