At least 45 people were arrested in the US city of Seattle after a Black Lives Matter protest turned violent, police said.

The protest held on Saturday night was one of the most tense of several rallies held across the country, the BBC reported.

Thousands of protesters had initially gathered in the city peacefully, carrying signs such as "Feds go home" and "We are living in a police state", and shouting chants of "No justice, no peace".

A group then set fire to the construction site for a youth detention facility before smashing windows of a courthouse and nearby businesses, police said.

Authorities said rocks, bottles, fireworks and mortars were thrown at officers, and one of them was taken to hospital with a leg injury.

Police declared the demonstrations a riot and said they were investigating whether an explosive device was used against a police station.

According to the Seattle Police Department, 21 officers were also injured in the clashes.

Besides Seattle, protests also took place on Saturday in Louisville, Kentucky; Aurora, Colorado; New York; Omaha, Nebraska; Oakland and Los Angeles in California; and Richmond in Virginia, reports the BBC.

The demonstrations have been given renewed energy by violent clashes in Portland city between protesters and federal agents deployed by President Donald Trump despite opposition from local and state leaders.

Trump said he would send federal troops to other cities including Chicago.

Demonstrations against police brutality and racism erupted across the US following the death of George Floyd, an African American who died in police custody in Minneapolis on May 25.

Some of the demonstrations have triggered violent clashes with police, vandalism, arson and looting..