An estimated 4,000 travellers had already gathered as trespassers on the land of a farmer, Stanley Pugh, before police from the Dyfed/Powys and West Mercia forces were able to prevent the number being swollen by people who had attended a legal rave party at Castle Donington, Leicestershire, on Saturday night.
Mr Pugh said he plans to take out an injunction to force the hippies to leave his land. Travellers' dogs are alleged to have killed a score of his sheep and fences were torn up for firewood, but one traveller said that collections had been held to compensate the farmer.
Dyfed police refused to allow powerful sound equipment to be taken on to the site for a rave festival. Low volume music was played throughout the night.
A large number of ravers from Castle Donington and elsewhere in the Midlands attempted the journey into Wales when they heard about the event. But police stopped all vehicles approaching the area.
Convoys of old buses, caravans, battered cars and even a black cab were halted. Vehicles left unattended when their occupants tried to make their way on foot to the site were towed away by police.
A number of sound systems were impounded, with police maintaining that the noise potential could threaten to breach the peace. More than 70 people were arrested for offences, mainly drug-related, over the weekend.
The invasion was condemned by farmers in the remote and little-visited part of mid Wales.
Gwilym Thomas, spokesman for the Farmers' Union of Wales, said: 'There is a very big fear that farmers will take the law into their hands - there is a great deal of bad feeling about what the hippies are doing to the land.
'Farmers are virtually powerless once these people get on to their land so we are urging them to block off all approaches to their property.'
The Liberal Democrat MP Alex Carlile, whose Montgomery constituency includes the festival site, praised the police.
'Their action in handling a tricky situation so well is an example other forces could well copy,' he said after visiting the area with Eifion Pritchard, the assistant chief constable of Dyfed/Powys.
Mr Carlile, a QC, said he would be pressing the Government to introduce a new law of criminal trespass which would empower police to stop people entering land if they had reasonable suspicion that a breach of the peace could result.
A police helicopter monitored the area yesterday afternoon. Police put up notices around the site requiring the hippies to leave under the Public Order Act, and broadcast the same message from the helicopter circling overhead.
It is feared that when the travellers disperse, some may try to make their way south to Llanbister, also in Powys, where 12 months ago a free festival staged on common land led to angry confrontations with local people. Last week, farmers there spread tons of manure over the ground as a deterrent.
Police in Leicestershire reported 24 arrests at the outdoor rave party at Castle Donington on Saturday night, which attracted 25,000 people.
The organisers, had won a court appeal to allow the rave to go ahead after efforts by the local council to stop it.
Ninety residents rang a special noise hotline, set up by North West Leicestershire District Council, to complain. Almost 500 officers had been on duty at the event.
A police spokesman said: 'It went very well. Most people were very well behaved. Disruption to the local community was kept to a minimum.'
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