SEATTLE (Reuters) - Five Seattle police officers were injured and at least nine people arrested on Sunday (May 1) night, after unruly demonstrators hurled projectiles and Molotov cocktails and broke windows, the authorities said.

Protesters gather on May 1 each year to focus attention on labour and immigration issues, but demonstrators in cities across the United States also used the occasion to rally against police violence.

Seattle Mayor Ed Murray blamed Sunday's trouble, which he termed "senseless violence", on a "different crowd" from those attending a peaceful May Day immigration march earlier, but he did not give details.

It was "deeply regrettable that in a city that goes to incredible lengths to respect First Amendment rights, there are some who disregard our values and engage in senseless acts of violence and property destruction", he said later, referring to free speech protected by the US Constitution.

The Seattle police department said in a series of posts on Twitter that one officer had suffered a laceration to the head or face as protesters hurled Molotov cocktails.

After making arrests in several places, the police said they had contained one group in a parking lot and were slowly allowing small groups to disperse, but were maintaining a presence in the area.

The Seattle Times newspaper reported that police were using "blast balls" to disperse a crowd throwing rocks and bottles, and breaking windows, in a downtown neighborhood.

Earlier, the paper reported that police had used pepper spray several times to break up throngs of demonstrators.

The unrest mirrored violence at a Seattle May Day march last year when crowds threw bottles and wrenches at police, who responded with pepper spray and flash grenades.

Sixteen people were arrested and three officers hurt in that violence, which similarly erupted after a day of peaceful demonstrations to focus on workers' rights, immigration issues and police treatment of minorities.