Jeri L. Williams, the chief of the Phoenix Police Department, said at a news conference late Tuesday night that officers were attacked with bottles, rocks, and tear gas, and that two officers were being treated for heat exhaustion at a hospital.

She disputed the suggestion that officers were overly aggressive, saying they responded with tear gas and so-called pepper balls, which emit pepper spray, only after they were assaulted.

But some witnesses said that events unfolded differently, with protesters throwing a water bottle or two in the direction of the police, before the police fired tear gas into the crowd.

Mayor Greg Stanton, who also spoke at the news conference, said that the police had attempted to allow people on the streets of downtown Phoenix to protest peacefully, and that there had been no serious injuries. But he added that officials were going to examine whether the approach by the police was necessary.