AMSTERDAM (Reuters) - Officials warned of potential gridlock around Amsterdam on Tuesday after an electricity outage in much of the city and the surrounding area hit trains, trams and metros.

Grid operator Tennet said the outage was caused by an unknown problem at a high tension power station.

“We’re working very hard to fix it,” Tennet said on social network Twitter. “The center of Amsterdam is unfortunately without power.”

The country’s road authority warned of long jams, and police asked people to call ambulance, police and other help lines “only in case of emergencies”.