At one moment last night, parents were watching at least a dozen children cavort on a playground outside their Prince George's County apartment house. Then came the crash and finally the screams.

Shortly before 8 p.m., an automobile rode onto the sidewalk and into the playground at the Oak Ridge Apartments in the Riverdale area, critically injuring an 8-year-old boy and his 6-year-old sister, police and fire officials said.

Cpl. Diane Richardson, a county police spokeswoman, said the boy suffered massive abdominal injuries, and the girl was cut severely on the head.

However, Richardson and a fire department spokesman said the injuries did not appear to be life-threatening.

The brother and sister were taken to Children's Hospital in the District by ambulance from the scene of the incident at 56th Avenue and Kennedy Street, just east of Kenilworth Avenue.

None of those involved in the incident was identified immediately. Police took the driver into custody.

Authorities said the children live in the garden apartment complex and the driver also lives in the Riverdale area.

The incident, at the end of the day on which county children returned to school after summer vacation, left bystanders and witnesses stunned.

"It was horrible," said one neighbor, who said she felt uneasy about being identified by name.

The boy was pulled from under the car by bystanders, according to a county fire department spokesman, Mark Brady.

The child was bleeding from the mouth as his mother held him, the neighbor said.

"She was screaming hysterically," the neighbor said of the mother.

According to the neighbor, the girl, who apparently was less severely injured, was taken from the playground into an apartment to await rescue workers.

Richardson said, "There were a lot of people out there" at the time the driver, who is 27, began trying to park in a lot at the complex.

She said he lost control of the late-model Acura, which climbed over a curb and kept going.

Richardson also said that police were trying to determine why the incident happened but that the cause was not immediately clear.

Witnesses said the car and the dark green playground equipment appeared heavily damaged by the crash.

The neighbor who saw the aftermath of the crash said the equipment had been placed there at the beginning of the summer and was put to good use.

She estimated that on a typical summer evening, more than 15 children would be there, sliding, climbing and swinging.