A major power failure has knocked out the electricity for more than two million people in southern California, parts of Arizona and northern Mexico.

Officials blame a transmission line in Arizona which automatically shut down two reactors at the San Onofre nuclear power plant near San Diego in California.

They are investigating the cause for the failure, but so far, they have ruled out terrorism.

The blackout has cut power to much of San Diego, parts of Arizona, and the states of Baja California and Tijuana in northern Mexico.

Power went out just before Thursday afternoon's rush hour, causing heavy traffic congestion and shutting down streetcar service. Radio and television stations were knocked off the air and flights at local airports grounded.