What started as a peaceful anti-war protest suddenly turned violent after demonstrators ignored a police order to disperse.
More than 60 people were arrested outside the site of the NATO summit and at least 12 others were said to have been injured.
Around 3,000 people attended the march, among them 50 American military veterans. At one point they threw their service medals into the street symbolising they said, their rejection of US-led wars in Iraq and Afghanistan.
The Chicago-based civil rights leader, Rev. Jesse Jackson walked with them.
“Those marchers are marchers of conscience. They must be heard, they ought to be heard. I hope that President and the NATO leaders will take in count the legitimate pain of the people here today,” said Rev.Jackson.
The rally was the biggest so far in a week leading up to the Nato summit. The Coalition Against NATO-G8 were the organisers but other protesters including some from Occupy Wall Street were also there.
Euronews reporter Paul Hackett said: “Those people here today are from different groups but they have the same message : No to war and no to NATO.”