Germany pushed on with efforts to secure river dykes with sandbags Thursday, bracing for a surge of the worst floods in over a decade that have claimed 12 lives and forced mass evacuations across central Europe.

Vast stretches along the Elbe river basin have turned into a sea of brown water in the Czech Republic and downstream in eastern Germany, with red-tiled roofs sticking out of the muddy water in many abandoned villages now accessible only by boat or helicopter.

The picture of devastation was similar along the mighty Danube, which has jumped its banks in Germany's southern Bavaria state and Austria and sparked large-scale disaster preparations in Hungary, where the water was expected to peak in coming days.

In northeast Germany, thousands of volunteers, many organised through social media, as well as 85,000 firefighters, aid workers and troops have filled millions of sandbags to hold back the torrent which has risen from two to above eight metres (six to above 26 feet).

Thousands kept a nervous watch on flood barriers while recalling dark memories of the 2002 floods that killed scores across central Europe and caused a clean-up bill running to billions of euros (dollars).

Fears were centred on Bitterfeld in Saxony-Anhalt state where two lakes, one higher than the other, loom dangerously close to a city that during the communist East Germany era became notorious as a heavily polluted industrial centre.

Local officials have warned that a breach in the lake defences could spark a "mini-tsunami", and officials have twice attempted to blow holes in the lake dyke away from the city, with limited success.

"I'm staying," said 77-year-old lakeside resident Joachim Grollmitz. "I won't let them force me out. We still have electricity and water and can stay informed about the water level."

But he did admit to AFP to some concern about a possible breach between the two lakes, saying that "then a big wave would come and roll over us".

Chancellor Angela Merkel was headed to the city Thursday, the second trip to a flood-hit region this week for the national leader who faces elections in September.

On a visit to Bavaria on Tuesday, Merkel promised 100 million euros ($130 million) in immediate flood relief. Finance Minister Wolfgang Schaeuble said Thursday that more money would follow.

Amid other offers of help from groups and businesses, carmaker Opel offered to lend flood-affected customers the free use of one of its cars for up to three weeks.

Dresden, with more than two million people, said the peak of 8.75 metres was reached on Thursday, with flood waters lapping through the mud-caked living rooms and trashed gardens of thousands of outlying homes.

However, the old inner city -- dubbed the "Jewel Box" for its baroque and rococo churches, opera and buildings -- was secured by flood barriers installed after the even higher 2002 flood.

People also breathed a cautious sigh of relief as water levels eased in Halle, where Elbe tributary the Saale had reached its highest level in 400 years the day before and authorities have urged 30,000 people to flee.

Upstream in the Czech Republic -- where five days of flooding killed at least eight people and forced some 20,000 evacuations -- rescue workers in rubber dinghies were supplying isolated families who lack drinking water, power or gas.

In the industrial centre of Usti nad Labem near the German border, where 11,000 people were told to evacuate, looters targeted empty homes and businesses, and a waiter at a pub-restaurant told how he came face to face with three robbers at night.

"I entered the corridor and got a blow. They broke my nose, my side is sore and there's something wrong with my ribs," Ladislav Kratochvil told the DNES daily.

In Austria, where two people have died in the floods, the Danube town of Korneuburg just north of Vienna reported an all-time record river level of 8.06 metres.

Down the Danube in Hungary, preparations moved into high gear to prepare Budapest for the wall of water coming along one of Europe's longest waterways which empties into the Black Sea.

Prime Minister Viktor Orban has warned large-scale evacuations were likely because of "a real threat to human life" but has pledged that "with good cooperation, we can protect everyone".

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