Police have fired pepper spray to disperse crowds in Phoenix, Arizona, after protesters outside a rally by US President Donald Trump threw rocks and bottles at officers.
"People in the crowd have begun throwing rocks and bottles at police. They also dispersed some gas in the area," Phoenix Police Department spokesman Sergeant Jonathan Howard said.
"Police have responded with pepper balls and OC (oleoresin capsicum) spray in an attempt to disperse the crowd and stop the assaults."
Police have not given an estimate of the number of protesters who turned out for the event, but Arizona media said there were several thousand people.
Four people were arrested during the protest, three on assault charges, Phoenix Police Chief Jeri Williams said during a news conference.
"We had tens of thousands of people downtown peacefully exercising their first amendment rights," Williams said.
"What's unfortunate is that a very small number of individuals chose criminal conduct."
Police called on the crowds to disperse. Many of the protesters quit the scene, while dozens of police in riot gear and carrying shields sought to clear remaining protesters from the downtown area.
A Reuters witness saw at least one person sat on a sidewalk, in handcuffs.
The Phoenix Fire Department said it treated 56 people for heat exhaustion and dehydration at the convention centre. Twelve people were taken to the hospital.
