A strong police presence brought relative calm to the Midwestern US city of Kenosha Thursday after three nights of violence over the police shooting of a black man, as top sports leagues again called off matches in a protest against racism.

The shooting of Jacob Blake during a confrontation on Sunday has added new fuel to demonstrations over police violence, with fresh turbulence spreading to Minneapolis and Los Angeles.

Ahead of his speech at the Republican National Convention Thursday, officials of President Donald Trump's administration condemned the protests as uncontrolled violence.

Protesters raise their fists during a demonstration against the shooting of Jacob Blake in Kenosha, Wisconsin in a fourth night of protests
Protesters raise their fists during a demonstration against the shooting of Jacob Blake in Kenosha, Wisconsin in a fourth night of protests AFP / Kerem Yucel

Vice President Mike Pence rejected on Wednesday allegations of endemic police racism and painted the protests as a threat to Americans, condemning "the violence and chaos engulfing cities across this country.

Trump is expected to deliver a fiery speech focused on "law and order", while many African Americans were preparing to join a protest in Washington Friday against police abuse.

Demonstrators march in the streets of Kenosha, Wisconsin for a fourth night
Demonstrators march in the streets of Kenosha, Wisconsin for a fourth night GETTY IMAGES NORTH AMERICA / Brandon Bell

Rights activists Al Sharpton and Martin Luther King III, Blake's family are to lead a "Commitment March: Get Your Knee Off Our Necks" protest in Washington -- a reference to George Floyd, who was killed in Minneapolis by a police officer who held a knee to his neck for more than eight minutes until he expired.

Police in Kenosha tried to encourage a calmer night on Thursday, urging people to respect a curfew.

"I thank those that peacefully protested and obeyed the curfew, not losing their voice, their desire to change to the violence," city police chief Daniel Miskins said.

A protester sits in the street while holding a placard during a demonstration against the shooting of Jacob Blake
A protester sits in the street while holding a placard during a demonstration against the shooting of Jacob Blake AFP / Kerem Yucel

"The voice of those people is not falling on deaf ears. We are hearing what is being said."

He spoke days after violence erupted on the city's streets in which an armed teenage vigilanted shot and killed two protesters.

The 17-year-old, reportedly a Trump fan, was arrested Wednesday on murder charges.

For a second day, major sports teams and leagues called off games and matches to keep the focus on the issue of racism and police abuse.

A reporter sits beside an empty court after the Milwaukee Bucks walked out of an NBA playoff game against the Orlando Magic to protest the shooting
A reporter sits beside an empty court after the Milwaukee Bucks walked out of an NBA playoff game against the Orlando Magic to protest the shooting GETTY IMAGES NORTH AMERICA / POOL

The NBA cancelled playoff games on Wednesday after the Milwaukee Bucks, whose base is near Kinosha, walked out.

"Change doesn't happen with just talk!! It happens with action and needs to happen NOW!" tweeted NBA star Lebron James.

The WNBA women's basketball league also postponed its slate of games on Thursday for a second day running.

In tennis, the ATP and WTA Tours announced that Thursday's play at the Western & Southern Open in New York had been called off, with the event resuming on Friday.

Several NFL teams meanwhile also halted training camps on Thursday as they build towards the start of their new season.

Activists continued to demand action against the Kenosha police officer who shot Blake, 29, in the back several times on Sunday as he tried to enter his car, with his three sons watching.

On Wednesday the Wisconsin Department of Justice said police were attempting to arrest Blake, though not specifying any charges, and that an attempt to tase him had failed. They added that a knife had been found in his car.

The federal Department of Justice meanwhile announced an FBI civil rights investigation into Blake's shooting, which has left him paralyzed from waist-down.

Illinois police were preparing to extradite to Kenosha 17-year-old Kyle Rittenhouse, who was detained on charges of first degree murder in relation to the shooting of protesters on Tuesday night.

Kenosha protesters said they were determined to keep marching for justice and police reform.

"We are targets. I'm big and black, they see me as an enemy all the time, everywhere I go," said a musician who goes by the name Big Homie Trail late Wednesday.

"Everybody's expecting us to go out all rage, to go crazy, in the fourth night but we are doing a peaceful protest as we are supposed to do," he added.

"We're going to keep marching, they are trying to shut us down, but they won't."