A spokesman for Finland's Kemira, which owns the plant, said 11,000 tonnes of sulphuric acid had leaked during the loading of a ship before 0400 GMT.

"In practical terms this means something like 300 truck loads of sulphuric acid," Kemira's Timo Leppa said.

Three hours later authorities in Helsingborg, which is 600 km (370 miles) south of Stockholm and 60 km from Copenhagen, said residents could safely leave their homes.

Police said six people were taken to hospital with respiratory problems but there were no fatalities. The port area was closed off and all traffic, including ferries, stopped.

Helsingborg has a population of some 85,000 people and is less than 5 km over the Oresund straits from the Danish city of Elsinore.

Some of the acid spilled into the sea starting a chemical reaction during which the water became very hot and produced vapour, Leppa said.

By 0700 GMT Helsingborg authorities allowed inhabitants to leave their homes, saying the noxious cloud was not moving towards more densely populated areas but remained within a 1 km radius of the cordoned off Kemira plant.

"The order to stay inside has now been lifted. Measurements have been taken which show that the toxic cloud is holding around the Kemira plant area," said fire inspector Ronny Moller.

Kemira shares opened 0.5 percent lower at 10.2 euros in Helsinki at 0800 GMT.