CHICAGO — Protesters at the NATO summit clashed with the Chicago police late Sunday as officers tried to disperse thousands of people who had gathered several blocks from the site of the meeting and refused demands to leave.


The standoff, which lasted several hours, grew intense as police officers, some in riot gear and gas masks, and protesters, some wearing all black, confronted one another, and shoving and scuffles broke out.


Some among the hundreds of police officers waved night sticks, and some protesters threw red paint, a bucket, sticks and more. By late afternoon, some protesters could be seen being carried away by officers, who tried to disperse the crowd with messages over loudspeakers, then seemed to push them back in waves with the sheer numbers of officers.


As the clash went on, mounted police officers could be seen equipping their horses with protective masks. At one point, an officer emerged from a scrum of people with his arm firmly around the neck of a protester, who appeared to be in distress. At another point, a demonstrator shoved an officer to the ground, before another officer pulled him to safety.


The confrontations came after a march through the streets downtown. Led by a small group of men and women in American military uniforms who said they wished to return their medals as a symbol, 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 a summit.


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 for the political protests that would accompany it. Early Sunday, the Chicago police said at least 18 people had been charged with crimes in connection with protests since Friday, but that number was certain to have grown by Sunday evening.


And by Sunday morning, 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. The 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, for 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.


Lawyers for both men denied the charges, and suggested that the authorities in Chicago were overstating the claims as a warning to the thousands of protesters, some of them linked to the Occupy movement, who have descended on the city for the summit meeting.


“It is very common for national security events to conduct these types of pre-emptive raids, pre-emptive arrests and attempt to spread fear and intimidation so you have fewer people out on the streets willing to protest and willing to risk violation of their constitutional rights,” said Kris Hermes, of the National Lawyers Guild, which has represented protesters here.


Law enforcement officials in Chicago dismissed such assertions, and said they planned to continue an approach that allowed for free speech and dissent, but did not tolerate crimes.


“Criminal behavior should be treated as criminal behavior,” the police superintendent, Garry McCarthy, told reporters on Sunday morning as demonstrations were beginning. “What you’re seeing is what we’ve been telling you’ be seeing for months now.”


The new arrests were unrelated, the authorities said, to a plot that prosecutors had earlier described involving three men they said had considered attacks against President Obama’s re-election campaign headquarters, the house of Mayor Rahm Emanuel, police stations and financial institutions.