Tension flared again on Sunday night, with one person shot and a police officer injured, in the Milwaukee area where the fatal shooting of a suspect by an officer had sparked rioting, prompting Wisconsin's governor to activate the National Guard.

Police violence against African-Americans has set off intermittent, sometimes violent protests in the past two years, igniting a national debate over race and policing and giving rise to the Black Lives Matter movement.

After peaceful vigils by small groups of demonstrators earlier, Milwaukee police said late on Sunday night they had rescued one shooting victim, who was taken to hospital. It was not known whether the injured person was a protester.