The flood led to an increase in congestion along eastern approach roads on Monday.

The underground is the sole rail service in eastern Helsinki.

Helsinki City Transport (HKL) said the flood damage might disrupt underground services for months, with trains terminating at Kaisaniemi instead of Ruoholahti.

Trains ran at five to six-minute intervals instead of the usual four on Monday.

HKL added that the central railway station underground platforms might remain closed for months.

HKL said it would investigate on Monday whether trains could pass the station safely in order to carry on to Kamppi and Ruoholahti.

"This is an extremely serious disruption for public transport as a whole," Matti Lahdenranta, the managing director of HKL, told the Finnish News Agency (STT) on Sunday as the extent of the disaster unfolded.

A replacement bus service named 65X runs between Sörnäinen and Ruoholahti, with extra trams on lines 6 and 8.

Helsinki Water said the flood had been caused by a burst 400-millimetre main.

Jukka Piekkari, the managing director of the city council-owned utility, said that while the cause of the rupture was not yet known, some of the pipes in the area were 70 years old.

Rescuers said no one had been injured in the flood. Hannu Arimo, a fire master, said the water pressure had reduced steel doors to "raisins".

Mr Lahdenranta said the damage would run to millions of euros. Overground railway services were not affected by the flood.

STT