More than 100,000 demonstrators are planning to disrupt the G8 summit this June in the German seaside resort of Heiligendamm — and the dress rehearsals have already begun.

German anarchists travelled to Copenhagen last month to give a violent edge to street protests against the closing of a youth centre. The visitors helped the Danes to erect blazing barricades and filled an armoury of petrol bombs. Then came a stand-off with riot police in Berlin — and more arson as the car of Thomas Mirow, the German deputy Finance Minister, was set alight.

Opposition to the G8 meeting on the Baltic Sea runs like a thread through all the incidents. The last big showdown before the summit is expected to be May Day in the German capital. Police are already calling it “a warming-up exercise”.

For years May Day in Berlin has been a moment of controlled anger when teenagers go on the rampage, looting shops and setting fire to police cars. This time it seems likely to have a political veneer. Leaflets show a picture of President Bush’s Air Force One next to a mangled wreck. The “autonomist groups”, anarchist cells across Germany, have called for a month of violence. “Destroy the image of your town,” says one pamphlet. “Riot against G8.” A music CD, meanwhile, entitled Move against G8, will be released after May Day.

“We have never before seen such a militant campaign of the extreme Left in northern Germany,” says Claudia Schmid, head of the Berlin wing of the Agency fo the Protection of the Constitution, the equivalent of the Special Branch. Similar reports are coming from Denmark, the Netherlands and the Czech Republic.

About 16,000 police, the most concentrated deployment since the war, have been drafted from across the country and will be backed by soldiers. A 13km (8 mile) steel wall topped with barbed wire is being built around the spa that once played host to the Russian Tsar, Felix Mendelssohn and Adolf Hitler.

Two US naval vessels will be moored off the coast, partly to intercept possible missiles, partly to monitor the water-front. A Royal Navy vessel is to help to patrol the 11km maritime security zone. The 6,000 journalists will be taken in and out of the red zone on a steam train, with armed guards.

“It will be a uniquely large and difficult operation, larger than the World Cup,” said Wolfgang Pistol, chief of police in Schleswig-Holstein. The two-day summit, entitled Growth with Responsibility, is costing close to €100 million (£68 million) for security alone.

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