US race protests: Protesters clash with cops

US police used flashbang grenades, pepper spray and tear gas as protestors marched in cities across the country on Saturday amid a wave of public anger over Donald Trump's planned "surge" of federal agents into major metropolises.

The demonstrations against racism and police brutality -- sparked by the death in Minneapolis of unarmed African-American George Floyd -- come as the US president faces an increasingly tough battle for re-election, and is campaigning heavily on a platform of "law and order".

Protestors marched in Austin, Texas, as well as Louisville in Kentucky, New York, Omaha, California's Oakland and Los Angeles, and Richmond in Virginia -- where riot police fired chemical agents at a Black Lives Matter march, according to US media.

In Seattle, the sounds of repeated small detonations rang out in some streets, and smoke rose from an area where demonstrators had set fire to trailers by a construction site for a youth detention facility, an AFP reporter observed.

Protestors slashed car tires and smashed trailer windows. Police in riot gear faced off against the protestors, some holding umbrellas against falling pellets of pepper spray.

Late Saturday, Seattle police said 45 people were arrested in connection with the demonstrations, which they designated a riot, according to the force's official Twitter account. The post also said "21 officers sustained injuries after being struck by bricks, rocks mortars/other explosives. Most officers were able to return to duty. One was treated at a hospital for a knee injury."

Police Chief Carmen Best implored people to "come in peace to the city," and castigated the demonstrations.

"The rioters had no regard for the community's safety, for officers' safety or for the businesses and property that they destroyed," local media reported her as saying.

In Texas, one person was killed when several shots were fired amid the Black Lives Matter protest in downtown Austin on Saturday, authorities said.

Footage posted during a Facebook Live showed the moment when several shots rang out in the Texas capital as about 100 people marched and chanted, "Fists up! Fight back!"

The latest violence came after police and federal agents fired tear gas and forcefully dispersed protestors further south in Portland early Saturday, also in anger over Trump's heavily-criticised surge of security forces.

The city, the biggest in the state of Oregon, has seen nightly protests against racism and police brutality for nearly two months, initially sparked by Floyd's death.