Leaks from a Finnish nickel mine that have dumped toxic metals into the environment were plugged on Wednesday, the mine operator said.

"The leakage of the gypsum pond at the Sotkamo mine (around 500 km or 300 miles north of Helsinki) has stopped," the Talvivaara group said in a statement.

"Most of the water that had previously leaked from the pond is contained within the mining concession area by the newly built fourth safety dam. No further release of the leakage water outside the mine area is anticipated," it added.

Toxic levels of nickel, cadmium, uranium, aluminium and zinc have been detected in a nearby lake and river, said the Finnish Environment Institute, part of the Ministry of Environment.

But the effects on the region's waterways won't be fully known before the spring or summer, the agency said.

Samples from the Kivijaervi lake and Lumijoki river show that the concentration of toxic metals exceeds levels deemed dangerous to fish and other organisms.

The government said on Tuesday that 900 kilogrammes (2,000 pounds) of nickel had leaked from the gypsum pond towards a lake located south of the plant, while around 220 kilos of nickel had leaked to the north of the pond.

On Wednesday, 1,000 people demonstrated in front of the ministry in Helsinki, saying the government's response had been inadequate and calling for the mine to be closed.

Finnish authorities have asked Talvivaara to submit more information about the accident before deciding when operations at the mine can resume.