About 45 protesters were arrested on Sunday, and at least four officers were injured, according to the Chicago Police superintendent, Garry McCarthy. It was uncertain how many protesters were injured.

The confrontations followed a march through the streets downtown, the largest demonstration during days of protests that have led up to the NATO meeting here. Led by about 40 men and women in American military uniforms who said they wished to return their medals as symbolic gestures, thousands of protesters opposed to war and to NATO or motivated by other issues marched down Michigan Avenue, winding their way as close as they could get to McCormick Place, where world leaders were holding the summit meeting.

For weeks, tensions here have grown over what it might mean in Chicago to host the first NATO summit meeting held in a United States city outside of Washington and face the political protests that would accompany it. Even before the tense confrontations, five people had been arrested miles from the street protests, but with what prosecutors described as plans involving bombs and other devices aimed at disrupting the summit meetings. Those arrests seemed to put a nervous city still further on edge.

On Sunday, prosecutors announced that they had filed charges against Sebastian Senakiewicz, 24, of Chicago, accusing him of falsely making a terrorist threat by claiming that he had homemade explosives — hidden in a hollowed-out “Harry Potter” book at his house — that could blow up a highway overpass. No explosives were found in a search of Mr. Senakiewicz’s home, the prosecutors said.

Separately, Mark Neiweem, 28, who was also believed to be from Chicago, was charged with “solicitation for possession” of explosives or incendiary devices. Prosecutors said he had discussed making a pipe bomb with an associate. He wrote a list, the prosecutors said, of all the items he thought he needed for the bomb.