He said officers had been hit by batteries and bottles thrown by protesters during the day.
"You can't control what other people are going to do, but I can tell you our cops are doing a great job, and they're prepared," he said.
Security has been tight throughout the city, as the heads of state from about 60 countries began arriving to discuss the war in Afghanistan, European missile defense and other issues. As police gathered en masse on street corners, near parks and key landmarks, the city's streets remained largely vacant and many downtown buildings closed.
"It's strange because downtown is empty," said Gabe Labovitz, a 44-year-old economist out for a walk near his home. "The police presence is reassuring but unnerving."
Three activists who traveled to Chicago for the summit were accused Saturday of manufacturing Molotov cocktails in a plot to attack Obama's campaign headquarters, Emanuel's home and other targets. But defense lawyers argued that the police had trumped up the charges to frighten peaceful protesters away. They told a judge it was undercover officers who brought the firebombs to an apartment in Chicago's South Side where the men were arrested.