Protesters have clashed with riot police as they tried to march towards the venue of the NATO summit in the US city of Chicago.
The two sides were locked in a standoff for nearly two hours, with the police blocking the protesters' path and the crowd refusing to leave.
More than a dozen protesters were roughly pushed onto the ground, handcuffed and hauled away as members of the angry crowd threw water bottles, wooden sticks and other objects at police clad in riot gear, according to the AFP news agency.
News crews captured images of protesters bleeding from head wounds, but officials would not provide information about the number of protesters injured.
Police said 45 people were arrested and four officers suffered minor injuries.
"Our officers have shown dramatic restraint," Garry McCarthy, a police superintendent, told reporters.
While police were able to eventually disperse the protesters, the crowd shifted to the central business district where noisy scuffles continued as darkness fell.
The hours-long standoff began after an otherwise peaceful march led by a group of veterans who spoke passionately about their opposition to the war before tossing away their medals.
Officials said an estimated 3,000 to 5,000 people had marched in the city.
One man played a guitar and sang "Give peace a chance" while others around him chanted, "This is what democracy looks like!" and "The people yes! NATO no!"
"They're spending money on wars instead of school so we can't get into college because it's too expensive," Isabel Olivia, a high school student, said.
A tight security blanket has been thrown over Chicago, the adopted hometown of US President Barack Obama who previously served Illinois as a senator, which is hosting more than 50 world leaders for the two-day summit.
Four protesters were arrested earlier in the week on terrorism charges and accused of making petrol bombs and plotting to attack targets including Obama's campaign office and the home of Chicago mayor Rahm Emmanuel.