AMSTERDAM, Netherlands (AP) _ A rush-hour power failure struck large areas of the capital on Tuesday, causing huge traffic jams, police said.

It was not clear what caused the break in Amsterdam's electric power supply, but it occurred shortly after a nitrogen tank in a subway station started leaking at 5 p.m., according to a police spokesman who declined to be identified.

The nitrogen was used as a coolant in an electrical switching box, the spokesman said.

The outage affected about half of this city of 650,000, according to a radio report.

The city's main thoroughfares became clogged when hundreds of traffic lights failed, and cable television sytems went out.

Less than an hour after the failure, several areas were on line again, the police spokesman said.