Thousands of customers remained without power in San Francisco following a massive blackout that disrupted traffic, shut down transit stations and, at its height, left a third of the city without electricity on one of the busiest days of the holiday season.

Crews were restoring power to thousands of homes and businesses today following a blackout that left a third of San Francisco without electricity on one of the busiest days of the Christmas holiday season.

The Pacific Gas and Electric Company, California's largest investor-owned utility, has filed for bankruptcy protection.

The worst day in California's power crisis has partly been blamed on a warm snap.

California has been thrown into chaos as major US cities suffered "rotating" power cuts.