A blast ball grenade explodes near police during protests in Seattle on Saturday. Police and demonstrators clash as protests continue in the city following reports that federal agents may have been sent to the city. (File photo: Agencies)

Police in Seattle used flashbang grenades and pepper spray Saturday against protesters who set fire to construction trailers outside a youth jail, amid a wave of public anger over US President Donald Trump's planned "surge" of federal agents into major cities.

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

Demonstrators slashed car tires and smashed trailer windows.

Police in riot gear faced off against the demonstrators, some of whom held up umbrellas against falling pellets of pepper spray.

The Seattle Times newspaper said 16 people were arrested on suspicion of assault against officers, obstruction and failure to disperse.

The latest spasm of violence came after police and federal agents fired tear gas and forcefully dispersed protesters further south in Portland early Saturday, also in anger over Trump's heavily criticized "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 the death of African-American George Floyd at the hands of police in Minnesota. 

Portland is also a stage for the highly controversial crackdown by federal agents ordered by Trump - one that is not supported by local officials there, and which many say smacks of authoritarianism.

Civil unrest was not only confined to Portland. On Saturday, three members of a black militia were shot in Louisville, Kentucky at a Black Lives Matter protest, local media reported, citing police. Their injuries were not life-threatening.

The protest, to demand justice for a black woman who was killed by police as she slept in her home, drew members of the black militia and a rival far-right militia, with the heavily armed groups facing off while separated by riot police. 

The groups dispersed peacefully later Saturday afternoon.

In Portland, Friday's demonstration was mainly peaceful, with crowds playing music and dancing, blowing soap bubbles and setting off fireworks.

But it ended - like many before it - in a showdown between protesters and police, which escalated in a haze of tear gas and flashbang devices.

One group of protesters formed a line with umbrellas and makeshift shields to try to protect themselves, as at least two fires burned outside fences around a federal courthouse.

Tear gas was first fired around 11 pm. By 2:30 am police and federal agents were clearing the scene outside the courthouse with tear gas, pushing protesters back.