The conservation body, which owns and looks after the 5,000-year-old monument, said that although the stones had not been damaged and no one was injured, the situation was "too unpredictable".

A spokeswoman added that it did not want to risk the safety of a second group. "Certainly, after the events of last night, the groups and individuals we have been dealing with have been understanding, but obviously they are disappointed," she said.

Nearly 100 people taking part in a Stonehenge Peace Process walk from Woodhenge had been due to end up at the circle at about 7pm.

Stonehenge is a focal point for people celebrating the summer solstice

The 150 people allowed onto the ancient site in Wiltshire on Sunday, including Druids and astrologers, had to abandon their solstice rituals.

They were disturbed by about 200 New Age travellers, who broke down fences early on Sunday morning and rushed towards the ancient site. Some were seen dancing on top of the stones.

About 100 police officers with protective clothing and riot shields - some on horseback - arrested 20 people for aggravated trespass, two for assaulting police and one for drugs offences.

Robert Hall reports: "Tonight police are on the alert once more"

One policeman and a number of travellers are understood to have been injured in the disturbance. A security cordon has now been set up.

About 1,000 Druids, hippies, travellers and sightseers were at the site on Salisbury Plain to watch the start of Britain's longest day.

Andy Hollingshead, Superintendent of the Wiltshire Police, said authorities planned a large presence on Monday night to enforce the order barring anyone from getting near the site.

"We realise the stones are a tourist site of great beauty and this will obviously detract from this, but we have to take measures to prevent what happened this morning happening again," he said.

'Sex, drugs and rock'n'roll'

But Kevin Carlyon, head of the British White Witches, claimed he had warned police there would be riots at Stonehenge, and had decided not to visit despite having permission.

"We sent police eight pages downloaded from a Website, which talked of a big rave organised at Stonehenge," he said.

He criticised the revellers' behaviour, saying: "It should be a place of worship. It is not for sex, drugs and rock'n'roll."

English Heritage had believed that limited access would help defuse tension and prevent some of the trouble seen in recent years.

English Heritage's Director of Stonehenge, Clews Everard: "It's been a very grim night"

Its director of Stonehenge, Clews Everard, said a small group of people had ruined the organisers' hard work. She said they had wanted to make the last summer solstice celebrations before the millennium "special".

"These people were not celebrating the solstice," she said. "What they were doing had nothing to do with spirituality, they're just a minority that has ruined it for the majority."

But reveller Simon Sturrit, 31, said people who had gained access to the site had caused no trouble or violence.

Solstice at Seahenge

The youth hostel worker said: "English Heritage feels that people aren't allowed on the site but early this morning we all managed to gain access and just had a good time."

But some Druids did manage to welcome the sunrise at another prehistoric monument - the circle of oak trees known as Seahenge. It recently emerged from the sea off the Norfolk coast.

A dozen "earth people" sang sacred chants while one of them drew a diagram of the summer and winter solstices in the sand.

"We very much hope there will not be more problems," the English Heritage spokeswoman said.