Stones and bottles hailed down on the German police last night as they tried to prevent thousands of anarchic demonstrators staging a dress rehearsal for a massive anti-G8 protest in June.

The mood was tense in Berlin as the peaceful rally against global capitalism turned surly.”Go back to your sties, you pigs!” chanted the swelling crowd as a group of hooded youths shot firework rockets and flare guns into a phalanx of police.

Dozens were arrested, thrown to the ground and handcuffed but all the indications were that the rioting would rage for most of the night. Ash particles scattered around the capital as rubbish containers were set ablaze.

Up north, in the Baltic spa resort of Heiligendamm, a 12-kilometre barbed wire fence is being erected to keep the leaders of the industrialised world at a safe distance from what is being billed as the biggest anti-globalisation protest of recent times.

But in Berlin last night there were no fences. Instead some 6,000 policemen were camped in the run-down district of Friedrichshain-Kreuzberg, waiting for the gathering tribes of the anti-G8 movement to make their move.

A newly created plainclothes unit called Intelligence and Intervention pinpointed protest organisers who intend to take their special skills to the summit on June 8. And the uniformed police sunning themselves before the night shift yesterday were reconciled to the likely clashes in Heiligendamm. ”We’re all going to the seaside,” said one officer, sweating in full battle gear.

There have been riots in Berlin on Walpurgis night, the eve of May Day, for the past twenty years when squatters set a supermarket ablaze, looted dozens of shops and overturned the police vans known, in Berlin slang, as bath tubs. Walpurgis night is traditionally the moment when winter is killed and spring is ushered in: reputedly witches take to their broom-sticks.

The modern version of this rite is to torch Porsches and smash restaurants, a kind of annual uprising against the wealthy and the privileged which often leads to hundreds of arrests and injuries.

This year, with protestors excited about the prospect of a huge show-down at the G8 summit, there was an even more violent edge to the night. One leaflet circulated by a group calling itself the Anti-Fascist Action,was calling yesterday on protestors to bring with them “Paint, stones, fireworks and butter-acid.” Butter-acid refers to butanic acid which can blind if thrown in the eyes. It smells of vomit (or rancid butter) and it is supposed to be poured on the leather seats of parked sports cars to make them undrivable.

Police warned that any use of fire-bombs or acid would lead to detention that would stretch well beyond June - that is, if the demonstrators behave badly this week they will miss the big event in Heiligendamm.Over the past weeks, the police have been touring schools to give 478 anti-confrontation lectures.

But many of the hooded youths who gathered in the hotspot of Boxhagen square yesterday evening were from outside Berlin; some were speaking French; others Dutch. Their idea seems to be to link up with the simmering discontent of the local Turkish migrant community. Until midnight their demonstration -Youth Against Capitalism - was deemed legal. But the crowd was joined by a marching legion of ant-globalisers and if, by Tuesday morning, they are still on their feet they will merge into another protest staged by the Revolutionary First of May group.

”Perhaps it won’t be so bad,” says Gueney Yilmaz selling beer at the Boxi-kiosk. He was one of the few shop-owners ready to stay open as dusk fell. Police ordered him to stop selling beer in bottles that could later be used as petrol bombs and some of Mr Yilmaz’s confidence deserted him. The steel shutters were rolled down and his brothers took up position outside the door.

Rarely has May Day - usually a time of disciplined outings by whistle-blowing trade unionists drumming up support for higher pay - been so nervous in Germany, so edgey.

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