Thousands of NATO protesters demanding an end to costly and destructive wars marched in Chicago Sunday where the leaders of more than 50 nations were discussing the withdrawal from Afghanistan.

Despite sweltering heat and looming thunderstorms, the marchers set off banging drums and blowing bubbles heading for the lakefront convention center where the two-day summit was being held.

They were joined by a group of veterans, many dressed in combat fatigues, who said they were marching for peace and planned to turn in their medals.

Kevin Cunningham, a Chicago carpenter, stood with a kite he'd picked up at a NATO sponsored event for children on which he'd scrawled "fly kites not drones."

"I was shocked and perplexed why a military organization was handing out kites to kids -- it seems a little Orwellian to me," he told AFP.

The father of two said he thinks NATO needs to be disbanded and said taxpayer funds spent on pointless wars would be better off funding education and health care.

"I'm against the war," said Jean Anderson, 41, a protester from Minnesota who held a sign declaring "1984 was not supposed to be an instruction manual" as she waited in a long line for water.

"There are powers that I need to fight back against and I need to represent people who can't speak out because they're in oppressed nations."

Officials from the mayor's office estimated the crowds gathered so far on the streets of the Windy City as between 1,800 to 2,200 strong.

"I've been told that it resembles more of a championship sports game than a protest, so yes everything seems quite peaceful," Bill McCaffrey, spokesman for the mayor's office, told AFP.

High school student Isabel Olivia,18, joined a Code Pink contingent and played with a pink cardboard gun which declared "NO NATO."

"We want peace, we want equality, we want more money for education," she said.

"They're spending money on wars instead of school so we can't get into college because it's too expensive."

Fears that demonstrations could turn violent have put Chicago on edge, with some downtown businesses boarding up their windows and others telling office workers to ditch their suits and ties to avoid being hassled.

So far the protests which have swamped Chicago for the past week have been largely peaceful.

But four protesters were arrested on terrorism charges and accused of plotting to toss Molotov cocktails and attack president Barack Obama's campaign office, police stations, banks and the home of Chicago mayor Rahm Emmanuel.

A police officer received a concussion after the van he was driving at the head of a march was swarmed and protesters hit him through his open window and slashed the van's tires late Saturday.

Two other men were arrested downtown after weapons and anarchist flags were found in their car and the city's website was hit by a cyber attack.

"I feel very good about the way the officers have handled these stressful events and I anticipate that today's going to be a tough day but we're going to have peaceful protests," Chicago police superintendent Garry McCarthy told reporters.

"It's a very big job, fortunately it's a big department," he added.

"We're well trained, we're well equipped and we're going to facilitate people's right to protest but we will not tolerate criminal activity."

Police and protest organizers have vowed that there will be no repeat of the trouble that erupted at G20 summits in London and Toronto or the riots that scarred Chicago during the 1968 Democratic National Convention.

Police have corralled the protesters using officers on bicycles and horses along with walls of blue uniforms to keep them away from the hotels housing world leaders but allowed them to march freely down the streets of the business district.

McCarthy said they will be relying on an 'extraction' technique to remove any trouble makers and allow the "99.5 percent" of the protesters who are peaceful to continue to express themselves.

Officers in riot gear were held in reserve and mostly out of sight in order to keep the overall mood from getting tense.