Police made 15 arrests as between 300 and 500 anti-war campaigners brought chaos to the centre of Brighton.

The protesters were demonstrating against possible military action in Iraq.

Some accused police of using heavy-handed tactics, including baton charges and pepper spray.

Alexandra Molano, 20, said: "We've come here to show our support for peace rather than violence and the only violence has come from the police.

"They suddenly started running towards us, hitting people with batons and spraying pepper in our faces. We were pushed out of the way.

"We have legal observers and we will be reporting how police officers treated us."

A police spokeswoman said: "The majority of people were peaceful and just wanted to make their point.

"But there was a minority who committed public order offences such as behaving aggressively and threatening assault."

The protest, which was organised by Sussex Action for Peace, started at 3pm yesterday when demonstrators gathered at the University of Sussex campus before marching towards the city centre, banging drums and carrying banners.

Police tried to confine the protesters to one lane of Lewes Road but, as the parade continued to grow, traffic was completely blocked. Demonstrators then gathered in Grand Parade before at least 200 occupied the roundabout at about 6pm, banging drums and shouting anti-war slogans.

They tried to block off all the roads and some lay down in the carriageway but police removed protesters and cleared all but two of the streets.

Protesters surrounded a motorist's van on the roundabout and a man hooked himself under the front bumper and had to be dragged away by police as a scuffle broke out.

Massed ranks of officers then manoeuvred protesters along Madeira Drive at about 7pm, before the crowds dispersed, only to reassemble on St James's Street, continuing to chant anti-war songs.

Well-wishers leant out of windows to cheer the march as it went past, though some revellers in the city for a night out were heard to complain about the disruption.

Protesters wearing George W Bush and Tony Blair masks or Hallowe'en make-up mingled with women in party frocks and sharp-suited men heading the other way into the city centre.

Miranda Royston, 20, said: "It just goes to show that people who protest against the Government are bound to be treated with contempt. This is a democracy but only as long as everyone falls into line."

Law students from Sussex University attended as independent observers.

One said: "We are here to make sure the police behave. We are giving legal advice and cards with people's rights if they are arrested."

A protester who saw his friend being taken away by officers said: "We're here to try and bring about peace.

"Maybe it has come to a point where the Government has gone so far but isn't listening to us so we have to take action. The Government is committing murders in our name and tonight is about showing the world we don't support that.

"It's my first demonstration, I've not done anything like this before.

"To be honest, I expected it would turn out like this, with arrests and violence. I'm not sure it's the best way of achieving peace.

"The antagonism was inevitable. We want to take action against the war and the police have a job to do and need to get people home."

Police officers raised their batons when a roundabout opposite the Palace Pier, Old Steine, was occupied.

They pulled people from the crowd, dragging and carrying them to waiting police vans.

Saul Lovell, 21, a student at the University of Brighton, said: "My friend was arrested for sitting on the roundabout. The police all stormed forward and there was a scuffle.

"He was sitting down when a policeman hit him round the head with his hand, smashing his glasses."

Paul Deacon, 39, from Worthing, said he was sprayed in the eyes by police. He said: "I was sitting down peacefully in the road and the police started trying to drag people off from the edges of the group so we all held onto each other.

"I was holding somebody and suddenly a policeman sprayed me in the face. I was blinded for about 20 minutes and I'm stinging all down the side of my face from it.

"I asked the police for medical assistance but they wouldn't let me have any."

A mother, who would only give her name as Anna, stood on the outskirts of the melee with her four-year-old son. She said: "I think the police have been really aggressive right from the start. People have the right to voice their opinions.

"Because of having my son here, I've stayed on the outskirts and I've seen the way the police have been behaving. They've dragged quite a few women inside the vans."

Fiona Phillips, of Sussex Action for Peace, said: "When we occupied the roundabout the police started raising their batons towards us.

"I saw people getting smashed in the face. We were moved back but it took an hour for them to get us moved away.

"I think people have got the message, which is war cannot be the answer. The majority of people in this country are opposed to George W Bush and Tony Blair attacking Iraq just for the sake of oil and to boost their own New World Order."

Group member Jenny Swindells said: "Over the last 12 years a million people have died as a result of US and UK bombing and UN sanctions. This cannot be allowed to continue."

Participants said they had been taking part in non-violent, direct action workshops to prepare for the protest. The event had been organised as part of a nationwide campaign of civil disobedience, called for by former Government minister Tony Benn.

University of Sussex student Debbie Fuller, 22, said: "The message is that war is not the answer. This shows the amount of support there is for peace and how many people are determined to tell Tony Blair and George W Bush, do not declare war in our name.

"Police have been kicking people off their bikes. They wanted to try to confine the parade, but they had to back down."

A police spokeswoman said: "We are trying to enable a peaceful protest to happen. that's their right. We are here to make sure it's lawful and peaceful and to make sure there is no additional disruption to the city.

"About 90 demonstrators gathered at Sussex University at 3pm. They began walking into the city and their numbers grew.

"Police officers walked alongside the protesters, so they did not walk into the paths of passing cars. But for about 15 minutes, the A270 was closed in both directions.

"By 6pm the protesters had reached a grassy area near the war memorial and a group broke away and sat on the roundabout near the Palace Pier. Hundreds of others then followed and blocked the road.

"People are allowed to protest peacefully and the police know that, as long as they do not endanger others or cause an obstruction. People cannot just go and sit in the middle of a road."

Chief superintendent Doug Rattray said it was possible officers, at their own discretion, may have had to use the spray canisters they routinely carry, on individuals. He said batons were not used to his knowledge although he said they may have been seen.

Although he could not confirm whether the batons had actually been used, he said they were not used as part of a mass police tactic on a group.

Among the demonstrators was Keith Taylor, Green Party councillor, who said: "I am here to mark my deep concerns and those of the Green Party at the possibility of invading Iraq."

Richard Mullin, a member of the Brighton and Hove Socialist Party, said: "This war is about America wanting control over the oil supply."

Gerald Clark, 73, from Worthing, said: "I feel very indignant about the war-like policies being pushed upon us. If there is going to be a war, it's a good thing to keep protesting like this."