A demonstrator gestures during a protest before the start of a NATO summit in Chicago May 19, 2012. [Photo/Agencies]

CHICAGO - Chicago police trying to keep the peace during the NATO summit may face their biggest test on Sunday when thousands of demonstrators were expected to march near the site where leaders of the military alliance begin a two-day meeting.

Previous protests in the runup to the summit Sunday and Monday have been lively but peaceful, resulting in fewer than two dozen arrests over the past six days, according to the Chicago Police Department.

But terrorism charges against three self-described anarchists arrested in Chicago earlier in the week was a reminder that the threat of violence is out there.

"So far, the numbers have been underwhelming," said Jeff Cramer, a federal former prosecutor who now runs the Chicago office of the global security consultancy Kroll International.

"We can't say the police found the only three people who are bent on violence. Vigilance is the word of the day."

The Coalition Against NATO-G8, the group behind Sunday's parade, has said it hopes as many as 10,000 people will show their opposition to the war in Afghanistan by participating in the march, which starts in a downtown park and ends more than two miles away near the summit site.

"Sunday will be the day the protesters get closest to the summit, and it will be the day we see the largest number of protesters," Cramer said.

"There are certainly going to be arrests and maybe a scuffle or two. I would be surprised if there weren't."

Fears that violence would erupt have so far proved unfounded and there has been little destruction of property. Experts credited the low arrest numbers to restraint by both police and protesters.

Chicago Police Chief Garry McCarthy said at the beginning of the week that his goal was to "extract" those protesters who were provocative and let others demonstrate peacefully.

"They're dealing with individuals that are showing signs of aggression. But they're allowing people expressing their opposition to policies to do so," said Arnette Heintze, a former Secret Service agent who is now CEO of Hillard Heintze, a security firm that advised the NATO host committee.

The three men charged Saturday with conspiracy to commit terrorism were planning to attack President Barack Obama's campaign headquarters and Chicago Mayor Rahm Emanuel's home, prosecutors said.

Ben Rhodes, deputy national security adviser to Obama, said on Saturday he did not think the president had been briefed on the alleged plot and praised Chicago police for acting swiftly.

"If these more serious allegations are true, then I think it was effective work in making sure that they couldn't pose any additional threat to public security," Rhodes said.

Obama and representatives from some 60 countries are to discuss the war in Afghanistan and other international security issues.