CHICAGO (ABC News) -- Protesters trying to push their way through a police line clashed with officers at the end of the anti-NATO march near McCormick Place.
Slideshow: Protesters, officers clash during NATO Summit protests
The earlier rally and march were peaceful, police said, and one group is believed to have incited the riot.
A group of protesters dressed all in black and believed to be part of the Black Bloc threw items at officers and hit them with sticks. Police officers used batons to respond.
At least two injured protesters were treated by volunteer medics on the scene.
Chicago Police Superintendent Garry McCarthy was spotted in the middle of the melee. He was behind officers in full riot gear. Those officers -- and the mounted patrol-- were called in to disperse the crowd after the march and rally had ended.
Officials say the Black Bloc is a group of protesters that attend events around the world creating civil disobedience. Earlier Sunday, seven people dressed in black were arrested for allegedly having items that could be used as weapons.
“You have classic Black Bloc ideology, peaceful ceremony, moving ceremony and these individuals use this as an occasion to disrupt, engage the police, engage in criminal activity. Once they crossed that behind and are throwing bribes at the police officers and hitting them with sticks and weapons then they have no option but to maintain control. It is classic Black Bloc ideology. It ruins ceremonies and ruins a ceremony of veterans turning in their valor medals," former Chicago police superintendent Jody Weis said Sunday.
The Black Bloc was also believed to be involved in another incident that took place during the march, an Associated Press report indicated that police and protesters clashed during the middle of the parade. The protesters, dressed in black, allegedly surged toward police and threw objects at officers. Police fought back with batons. Those protesters ran away, but reconvened with the main crowd near the parade's end at Cermak and Michigan Avenue and are believed to be part of the later clash with police, who beefed up security around that area -- and officers showed up in full riot gear.
There is no official count of how many people were arrested during the protest, but there are reports of several arrests.
Police say for the most part the NATO protests have been peaceful. The protesters had permits for Sunday's Grant Park rally and parade, which was expected to be the largest anti-NATO event during the Chicago NATO summits. As many as 2,000 people attended, officials said.
The anti-NATO march was led by dozens of U.S. military veterans, mostly Army and Navy, who have served tours in Iraq and Afghanistan.
Over the week, the vets have shared their stories of disenchantment about a mission they say changed from disarming a country, liberating a country and then occupying that country. They're in town to argue on an international stage that troops should be withdrawn from Afghanistan before 2014.
"For every house that we enter, if there weren't terrorists there before, there are now," Graham Clumpner said.
"The military values the mission over the lives of these soldiers who recognize a very legitimate moral dilemma," Brock McIntosh said.
Also, they want full mental health care benefits for veterans.
At the end of the parade route, veterans threw their NATO medals over the fence that the Secret Service put up as a perimeter around McCormick Place. Each of them spoke briefly.
"We don't want your medals, we want to give them back because these are dishonor to hold, we want to give them back because we stand in solidarity with the people of Afghanistan, Iraq and all victims of occupation around the world," Scott Kimball said earlier Sunday.
"As veterans we swore to protect. We are once again going to step out front,make ourselves vulnerable and represent the majority of the people in this country. And give our medals back," Jacob George said.
The veterans had hoped a NATO representative would meet the group and take the medals back as a symbol of recognition.
Sunday's rally and march was organized by Coalition Against NATO/G8 Rally/ March, but dozens of other groups are also joining, ranging from Occupy to individuals.
"We believe there is no more real usefulness for NATO. It is no longer a form of defensive role. NATO should be eliminated," Blaise Sewell, Occupy Chicago, said.
"So yesterday we saw a massive amount of people mobilizing on the streets and standing up for their rights and I think today we're gonna see that times ten," Lenny Bianchi, Veterans for Peace, said.
Sunday's planned rally and march follows one other permitted rally, which was held Friday in Daley Plaza by Nurses National United. That rally was peaceful. However, since then there have been several impromptu marches that tied up downtown streets, including one Friday afternoon and one Saturday evening, which involved multiple arrests.
Supt. McCarthy spoke about an incident in which a protester was pushed by a police van on Saturday night. He said the officer was injured, suffering a concussion, but the protester was released from the hospital.
"The officer was at the front of the march and unfortunately the march overran him," Supt. McCarthy said. "This was one officer in a van by himself being swarmed by a crowd. Significant damage to the vehicle, his tires were slashed, the officer was injured. He had to get out of there. It's really that simple."
For more on this and other stories, click on ABC News.com.