PHOENIX: Police fired pepper spray to disperse protesters outside a rally by US President Donald Trump in Phoenix, Arizona, on Tuesday after being pelted with rocks and bottles, police said.

Police have not given an estimate of the number of protesters, but Arizona media said there were several thousand.

"People in the crowd have begun throwing rocks and bottles at police," 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," he said.

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.