At least 45 protesters were arrested and four police officers injured on Sunday during protests outside the 2012 NATO Summit in Chicago, according to authorities.
Protesters and officers clashed at the McCormick Place convention center, the largest convention center in the United States, where the diplomatic forum was held.
The protests, attended by about 3,000 people marching against various grievances including the war in Afghanistan, climate change, and economic inequality, turned violent after city police called on protesters nearing the convention center to disperse or risk arrest.
Several hundred participants did not respond to the requests of law enforcement, and instead hurled sticks and bottles at officers, who responded with batons.
Lawyers assisting protesters have argued against the tactics used by police, including the use of batons, and are challenging police figures arguing that at least 60 people had been detained and 12 protestors injured, according to Reuters.
"I know that the picture (of officers in riot gear pushing and hitting protesters) is going to be what people are going to run away here with. But cops are not here to be assaulted," Chicago Police Chief Garry McCarthy said of Sunday's clashes.
The 2012 NATO Summit, where missile defense, defense modernization, and transitioning Afghanistan are the major topics of debate is being held May 20-21 in Chicago.
Nonviolent protests drawing small-to-moderate sized crowds started as early as last week and a group of around 50 veterans also attended Sunday's protests to return their war medals.
Members of the group Iraq Veterans Against the War (IVAW) threw their service medals into the street near the site of the convention center.
"We, Afghanistan and Iraq veterans, from around the country have united with CANG8 Coalition against NATO/G8 war and poverty agenda to converge in Chicago on May 20th for a unity march to the NATO summit and ceremoniously return our service medals to NATO generals," a statement on the IVAW website read.
"We were awarded these medals for serving in the Global War on Terror, a war based on lies and failed policies."
Protests are expected to continue into Monday at the Chicago headquarters of Boeing Corp. with demonstrators set to protest against the company's role in building planes for the U.S. military.