A dam has burst in south-eastern Germany as flood waters sweep northwards from the Czech Republic, spreading chaos in Europe's worst floods for more than a century.

The dam at Bitterfeld collapsed around midday, sending the Mulde River cascading towards the town, whose 16,000 inhabitants are being urgently evacuated.

A large chemical plant at Bitterfeld is unlikely to be flooded, the authorities say, but there are fears of environmental damage after a chlorine gas leak from a Czech plant near the German border.

The swollen Elbe is surging towards Dresden, where the water level is rising at around 20 centimetres per hour and many of its bridges have become impassable.

Magdeburg, north-west of Dresden, is braced for a state of emergency at the weekend when the waters are forecast to hit the city.

Residents in three districts have been told they must leave their homes by Saturday and seek refuge elsewhere. Schools in the city are being transformed into temporary shelters.

In the Czech capital Prague, Bavaria and Austria, meanwhile, the situation has eased as water levels have started to drop.

Elbe watch

The German Environment Ministry has set up an emergency team to monitor the Elbe for toxic leaks.

A cloud of chlorine gas escaped from a flood-damaged Czech chemical plant across the border near the town of Usti nad Labem.

A spokesman for the Spolana plant said the gas was being dispersed by wind but a third-degree chemical alert was issued to local people.

German towns are devastated as waters surge northwards

Fed by the Vltava, which flows through Prague, the Elbe is rushing through the north of the Czech Republic into the German state of Saxony.

Czech authorities blew up five barges after they broke loose from their moorings at the border town of Decin and threatened to career downstream towards Dresden.

In Dresden itself, the Elbe is already eight metres above normal and is expected to rise to levels not seen for more than 100 years.

The situation is becoming critical in Dessau, where the Elbe and the Mulde river merge. Volunteers are stacking sandbags along the banks after a sudden rise in river levels.

The towns of Magdeburg and Muehlberg are also on alert.

Workers are pumping water from Dresden's Zwinger Palace

Tens of thousands of people have been evacuated around the region and emergency teams have been sent from around Germany to shore up rescue efforts.

Workers have been pumping water from Dresden's Zwinger Palace art gallery, where precious paintings, including Rembrandts, have been moved to upper floors.

And nearly 200 patients from the intensive care unit of the local hospital have been evacuated by air and land to hospitals in other cities across Germany.

At least nine people have died in Saxony alone, which has been promised federal aid. A fireman died in Pirna while trying to rescue people trapped by the waters.

Clean-up

Prague's Old Town was spared when the Vltava river stopped just short of hastily-built flood defences on Wednesday but the Czech capital faces a clean-up bill of hundreds of millions of dollars.

Thousands of people are still unable to return home as electricity and sewerage supplies remain cut off, and the death toll has reached at least 11.

In some places the clean-up effort is under way

In neighbouring Slovakia, the capital, Bratislava, remains on high alert since the Danube river rose to almost 10 metres higher than normal - its highest level for a century..

A state of emergency is in force, people have been evacuated and the situation is being monitored by helicopter.

The river has already wrought havoc in Austria and southern Bavaria, where some cleaning-up efforts have already begun as the waters recede.

Seven people in Austria died in the floods.

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