KENOSHA (REUTERS) - A black man shot in the back by police in Kenosha, Wisconsin, was left paralysed and "fighting for his life," his family and lawyers said on Tuesday (Aug 25), hours before protesters and authorities clashed for a third night of civil unrest in the lakefront town.

Police in riot gear skirmished after dark with about 200 demonstrators defying a dawn-to-dusk curfew outside a courthouse and an adjacent park in the downtown area of Kenosha, a city of about 100,000 people about 60km south of Milwaukee.

Several military-style armoured patrol vehicles were seen manoeuvring around the county courthouse, firing tear gas at the crowd, many of whom hurled water bottles, firecrackers and other objects back at police.

The disturbances came hours after Wisconsin Governor Tony Evers declared a state of emergency and vowed to deploy additional National Guard troops in a bid to restore order in the town, while the mother of the man who was shot, Mr Jacob Blake Jr, publicly appealed for calm.

Mr Blake, 29, a father of six, was struck from behind at point-blank range in a hail of bullets fired on Sunday by police who were following him with guns drawn as he walked away from officers to his car and opened a door to the vehicle.

Three of his young sons inside the automobile - aged 3, 5 and 8 - witnessed their father being gunned down, according to civil rights attorney Ben Crump, who represents the Blake family.

A bystander captured the encounter in video footage that immediately went viral, unleashing outrage over the latest in a long series of instances in which police have been accused of using indiscriminate lethal force against African Americans.

Mr Blake, who had been attempting to break up a quarrel between two women, was struck by four of seven gunshots fired at him, all by one officer, and there was "no indication he was armed," Mr Crump said in an ABC News interview on Tuesday.

The police have not explained why Mr Blake was shot.

At a news conference later in the day, Mr Blake's parents expressed anguish over the shooting while decrying two previous nights of looting, vandalism and arson that overshadowed peaceful street protests.

'MY SON MATTERS'

"They shot my son seven times. Seven times! Like he didn't matter," Mr Jacob Blake Sr, his voice crumbling with emotion, told reporters. "My son matters. He's a human being and he matters."

Ms Julia Jackson, Mr lake's mother, said her son "has been fighting for his life," but called for unity, saying she was praying for police officers. She also said her son would disapprove of damage done to the city, declaring, "It doesn't reflect my son or my family."

Officials, blaming outside agitators or much of the lawlessness surrounding protests, said some three dozen fires were set on Monday night, mostly in an uptown, predominantly minority neighbourhood where numerous businesses were burned.

Seeking to restrict outside traffic into the city, authorities blocked off all exit ramps leading from Interstate-94 into Kenosha County from the adjacent city of Racine south to the Illinois border along Lake Michigan.

Traffic barriers and heavy fencing were also erected on Tuesday around a downtown cluster of public buildings, including the courthouse, police department and sheriff's headquarters, in an effort to protect them from arson and vandalism.

Mr Crump and another lawyer for the Blake family, Mr Patrick Salvi II, said bullets shattered some of Mr Blake's vertebrae, leaving him paralysed from the waist down - possibly permanently. He also suffered gunshot wounds to his stomach and gastrointestinal tract, requiring surgeons to remove most of his colon and small intestine, and sustained damage to his kidney, liver and one of his arms, they said.

"It's going to take a miracle for Jacob Blake Jr to ever walk again," Mr Crump said, adding that the family intended to bring a lawsuit "to hold wrongdoers accountable." Family and protesters are demanding that the officers involved in the shooting, who have been placed on administrative leave, be fired and prosecuted.

Video captured by a neighbour shows Mr Blake walking towards the driver's side door of his SUV, away from two officers who were pointing guns at his back. After he opens the door and leans into the car, seven shots ring out, with one of the officers tugging at his shirt.

The shooting is under investigation by the Wisconsin Department of Justice, which has not released any details.

Kenosha police have referred all questions to the state investigators.

The shooting occurred three months after George Floyd's death in Minneapolis sparked nationwide protests against police brutality and racism.