Los Angeles  - The death toll from the worst train crash in southern California history mounted late Saturday to at least 25 people, with 135 injured.

Officials blamed the driver of the passenger train for running though a red light and ploughing around a bend, head on into an oncoming freight train.

"Barring any information from the National Transportation Safety Board, we believe our engineer failed to stop, and that was the cause of the accident," Denise Tyrell, spokeswoman for the local transit authority, told the Los Angeles Times. "Of course, it is your worst fear that this could happen, that the ability for human error to occur could come into the scenario."

Tyrell was unable to explain why a series of safety measures and controls along the way failed to stop the train.

The horrific accident occurred at 4:23 pm (2323 GMT) Friday, when a freight train slammed into a packed commuter train on a stretch of single track near Chatsworth, in the San Fernando Valley north of Los Angeles.

Rescuers said that victims were piled up inside three mangled passenger cars. Working overnight, teams used hydraulic jacks to keep the passenger cars from falling over and other specialized rescue equipment to gently tear apart the metal.

"There's so much damage, we need to wait for heavier equipment," Fire Captain Steve Ruda told reporters at the scene.

"This is the worst accident I've ever seen," Los Angeles Mayor Anthony Villaraigosa said. "Clearly the injuries are going to mount, and so are the fatalities."

The Metrolink commuter train was packed on a Friday afternoon with an estimated 350 to 400 passengers as it entered the sharp bend where the collision occurred.

According to the operators, two drivers were manning the Union Pacific freight train bound for Los Angeles.

The Los Angeles Times quoted a retired engineer who had worked the route as saying that the passenger train should have waited on a siding for the freight train to pass. The trains were both using the same stretch of track because of narrow tunnels that run through the mountains around Los Angeles.

Television footage from the accident scene showed firefighters rescuing injured passengers from the wreckage of an overturned wagon. Debris from the freight train was strewn across the tracks.

Medical helicopters evacuated the most critically injured victims to nearby hospitals, and scores of ambulances drove to the site of the crash.

About 250 fire department personnel and 200 police officers were still on the scene Saturday.