Correction: An earlier version of this article referred incorrectly to a medal that Iraq veteran Mark Stach planned to return. The medal was issued by the United States, not NATO. This version has been corrected.

Thousands of peace activists thronged city streets to protest meetings of the North Atlantic Treaty Organization in Chicago on Sunday, a march for military veterans that was capped by a violent conflict with police.

Dozens of protesters stormed the police line after an emotional ceremony close to McCormick Place, where the NATO summit is being held. Protesters were beaten with police batons. One serious head injury was observed.

Police said that they had made 45 arrests and that four officers — including one who was stabbed in the leg — were injured in the confrontation. Several protesters were also hurt.

The day capped a turbulent week. Two more men were arrested on terrorism charges, including falsely making a terrorist threat and attempted possession of explosives, the Cook County state’s attorney’s office said. That’s in addition to the three Florida-based Occupy activists charged Saturday with plotting to throw Molotov cocktails at President Obama’s headquarters, the mayor’s house and police stations.

At least 19 protesters have been arrested in recent days; five were detained Saturday night after a series of clashes with police as they marched through the downtown Loop district.

Sunday’s march began relatively peacefully. Many demonstrators danced, chanted and sang, although the mood at the front of the column was more subdued. Three dozen veterans in battle fatigues solemnly chanted “No NATO, no war, we don’t kill for you no more.”

As they neared the convention center, they were greeted by rows of riot police, mounted officers, Jersey barriers and fences.

Many of the vets wept and apologized as they took off their medals and threw them down to the sound of cheers.

“These are lies. I’m giving them back,” said Jason Hurd, a former combat medic in Iraq.

Shortly afterward, the notes of taps were heard.

Leaders of Occupy Chicago held a news conference Sunday to denounce what they said was police violence. In one incident, a police van struck a protester during a melee late Saturday evening.

Police Superintendent Garry McCarthy said that the officer was punched through an open window of the vehicle and suffered a concussion.

“We face protests every day here, and we’re going to take care of business,” McCarthy said in a brief news conference in Grant Park. The police support the protesters’ right to demonstrate, he said, but “we’re not going to tolerate criminal behavior.”

More than 3,000 Chicago police officers and plainclothes federal agents were out as a no-fly zone went into effect over the city and U.S. Coast Guard boats patrolled nearby waterways and Lake Michigan.

Before the summit, hundreds of police officers received crowd-control training from the Department of Homeland Security and were outfitted with $1 million worth of riot gear. The police department is trying to avoid the sorts of violent confrontations that have marred the city’s history, from the 1968 Democratic National Convention to mass arrests during an Iraq protest in 2003. The city recently paid $6.2 million to settle a lawsuit over police conduct during the 2003 protests.

In Grant Park, hundreds of protesters gathered at the Pe­trillo Music Shell before the “Coalition Against NATO/G8 War & Poverty” march sponsored by a group called Iraq and Afghanistan Veterans Against the War.

“We’re demanding a full withdrawal of forces from Afghanistan, and we’re advocating bringing the incredible amount of money spent in Afghanistan back home to our communities,” said Matt Howard, 29, a former Marine corporal who is an organizer with the veterans group.

Mark Stach, 40, a veteran from Dixon, Ill., filled his canteen as he readied for the march to return a U.S. medal he received while serving in Iraq in the Army National Guard in 2004 and 2005.

“I’m willing to give it back, because I don’t think the war on terror did any good,” he said.

He added that he was enjoying the sense of community felt with other veterans.

“I feel pretty alone in this most of the time. I’m looking to be around people who feel the same way I do. I’m disgusted [the war] has gone on so long.”