BAGHDAD, Iraq, Sunday, April 3 - Using suicide car bombs and an array of weapons, scores of insurgents made the biggest assault yet on the American-controlled Abu Ghraib prison on Saturday evening, American military officials said. At least 20 American soldiers and marines were wounded.

Forty to 60 insurgents attacked the prison from opposite directions, but were repelled by the Americans in a pitched battle that lasted for 30 to 40 minutes, the officials said. They added that they knew of only one insurgent who had been killed, but said it was almost certain the guerrillas suffered additional casualties.

The assault appeared to be an attempt to break prisoners out of a part of the center that is controlled by Iraqi security forces, said Lt. Col. Guy Rudisill, a spokesman for the American detainee system in Iraq.

The assault was so intense that the American troops at the prison called in three Apache attack helicopters and a Marine infantry company, the colonel said. The marines quickly secured the area around the prison. Of the 20 Americans hurt, 18 had only minor wounds, Colonel Rudisill said.