At least 250 people were arrested outside the Republican Convention last night as police used tear gas and pepper spray to disperse rioters attacking property and blocking roads in protest at the war in Iraq.

They had come in their thousands – grandmothers, veterans, young families and even disgruntled Republicans bearing banners and peace flags, to demand an end to the five-year conflict. And for the most part, the demonstrations passed off peacefully.

But once the main antiwar march had finished, splinter groups embarked on a violent rampage, smashing windows, slashing car tyres, throwing bottles and even attacking Republican delegates attending the nearby Xcel Centre.

Many of those involved identified themselves to reporters as anarchists. These protesters, some clad in black, wreaked havoc by damaging property and starting at least one fire.

The Minnesota National Guard sent 150 soldiers to help police tackle the riots, which flared as delegates were assembling in St. Paul for the four-day meeting. The convention has got off to a slow and subdued start as Republicans turn their attentions to the millions in Louisiana who have fled Hurricane Gustav.

Members of the Connecticut delegation said they were attacked by protesters when they got off their bus near the convention centre, KMSP-TV reported. Rob Simmons, a delegate, told the station that a group of protesters came toward his delegation and tried to rip the credentials off their necks before spraying them with a toxic substance that burned their eyes and stained their clothes.

One 80-year-old member of the delegation had to be treated for injuries, and several other delegates had to rinse their eyes and clothing.

Terry Butts, a former Alabama Supreme Court justice who is a convention delegate, was on a bus taking delegates to the arena when a brick thrown through a window sprayed glass on him and two others. Mr Butts said he wasn’t hurt.

“It just left us a little shaken,” he said. “It was sort of a frightening moment because it could have been a bomb or a Molotov cocktail."

Five people were arrested for lighting a rubbish bin on fire and pushing it into a police car, St. Paul police spokesman Tom Walsh said. Of those arrested, 119 faced possible felony charges.

At least four journalists were among those detained, including Associated Press photographer Matt Rourke and Amy Goodman, host of Democracy Now!, a nationally syndicated public radio and TV news programme.

Ms Goodman was intervening on behalf of two producers for her program, Sharif Abdel Kouddous and Nicole Salazar, when she was arrested, said Mike Burke, another producer.

The anti-war march was organised by a group called the Coalition to March on the RNC and Stop the War, whose leaders said they hoped for a peaceful, family friendly event. But police were on high alert after months of preparations by a self-described anarchist group called the RNC Welcoming Committee, which was not among the organisers of the march.

Most of the trouble was in pockets of a neighbourhood near downtown, several blocks from where the convention was taking place. Police fired tear gas canisters and used pepper spray on protesters who tried to block key streets.

Up to 200 people from various groups marched in a noisy “Funk the War" march. Clad in black, protesters smashed windows of cars and stores, tipped over rubbish bins, pulled down street signs and bent the rear-view mirrors on a bus. Some wore gas masks and bandanas to protect themselves from smoke bombs and other chemical irritants.

The war was likely to get a second day of attention outside the convention on Tuesday as Ron Paul, a Congressman and former Republican presidential candidate who opposes the Iraq war, was expected to speak to supporters at a Minneapolis rally. Separately, a group advocating for the poor was planning a protest march toward the convention centre.

Security has been tight in St Paul, in a bid to avoid a repeat of the chaotic scenes seen at the 2004 Republican convention in New York when more than 1,800 people were arrested. Yesterday snipers looked on from nearby buildings and a helicopter hovered overhead as some 10,000 surged through the streets during the main march.

“We joined up with an altruistic vision of promoting freedom and justice around the world,” said Vince Emanuel, a Marine lance corporal who did a tour of duty in Iraq from August 2004 to April 2005, explaining why he was marching.

“Except we saw the killing of innocent people and the destruction of property ... for a lot of us it was very disenfranchising."

The veterans group tried to hand a message to the campaign of Republican White House hopeful John McCain calling for a withdrawal from Iraq, reparations for the Iraqi people, and full medical benefits for veterans. No one from Mr McCain’s campaign received them, Mr Emanuel said.

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