Civil unrest flared and curfews were imposed in several major US cities on Saturday as demonstrators took to the streets to vent outrage at the death of black man George Floyd.

Demonstrations have spread to cities across the United States after a video went viral of George Floyd being suffocated to death by a white police officer in the mid-western US state of Minnesota on Monday.

From Los Angeles to Miami to Chicago, protests marked by chants of "I can't breathe" - a rallying cry echoing the dying words of George Floyd - began peacefully before deteriorating into violence.

Demonstrators blocked traffic, set fires and clashed with riot police, some firing tear gas and plastic bullets in an effort to restore order.

By Saturday morning, broken windows and graffiti were seen as the crowds dispersed. Eight Houston police cruisers were damaged in the unrest, said reports.

Curfews were imposed in several major cities rocked by civil disturbances in recent days, including Atlanta, Los Angeles, Philadelphia, Denver, Cincinnati, Portland, Oregon, and Louisville, Kentucky. 

Los Angeles Mayor Eric Garcetti announced at a press conference Saturday afternoon that a curfew will be in effect in the city's downtown beginning from 8 pm Saturday to 5:30 am Sunday local time.

Seattle mayor Jenny Durkan on Saturday announced a city-wide curfew for Satruday and Sunday from 5:00 pm (2100GMT) to 5:00 am (0900GMT) after protests broke out downtown.