An electricity blackout caused by a failure at two substations near St. Petersburg affected most regions of Russia's second largest city, a spokesman for a local power grid said on Friday.

The blackout, which lasted for more than an hour, brought buses and subway and commuter trains to a standstill during rush hour.

Some 30 commuter and long-distance trains were delayed, while many passengers on St. Petersburg's subway - the world's deepest - had to walk along the tunnels and up the escalators to get out.

Traffic lights were off on Nevsky Prospekt, the city's central avenue, causing traffic jams. Almost half of the city, which has a population of 4.5 million, was left without water supplies.

Power supplies have been restored in all parts of the city, a spokesman for the power grid said, adding that preliminary causes of the outage will be known tonight or tomorrow morning.

However, the Emergencies Ministry said the city's historic center is still suffering from power cuts.

Local bloggers complained of failures of mobile and Internet communications comparing the situation with the 900-day siege during World War II.

This is the largest blackout in Russia since one in May 2005 that affected the Moscow Region. That power outage left more than 150 hospitals without electricity and caused major disruptions to commuter traffic. About 4 million people were affected.

ST. PETERSBURG, August 20 (RIA Novosti)