Protests have erupted in the US city of Kenosha, Wisconsin, after police shot a black man in the back several times from close range as he got into a car on Sunday night (local time), according to mobile phone video of the incident.

The man's three young sons were reportedly in the car at the time and witnessed the shooting, lawyer Ben Crump and the American Civil Liberties Union said. The man, identified by Wisconsin's governor Tony Evers as Jacob Blake, is in serious condition and fighting for his life.

Mr Blake was flown to Froedtert Hospital in Milwaukee and was in serious condition, Kenosha police said.

"Tonight, Jacob Blake was shot in the back multiple times, in broad daylight, in Kenosha, Wisconsin," Governor Evers said on Twitter.

"While we do not have all of the details yet, what we know for certain is that he is not the first black man or person to have been shot or injured or mercilessly killed at the hands of individuals in law enforcement in our state or our country.

"We stand with all those who have and continue to demand justice, equity, and accountability for black lives in our country."

US Democratic presidential nominee Joe Biden has said that the police shooting of Mr Blake, "calls for an immediate, full and transparent investigation and the officers must be held accountable."

"Equal justice has not been real for Black Americans and so many others. We are at an inflection point. We must dismantle systemic racism. It is the urgent task before us," Mr Biden said.

The two police officers reportedly involved were placed on leave.

Police said the shooting occurred when they were responding to a domestic incident at about 5:11pm local time (8:11am AEST).

Mobile phone footage of the incident shows a Black man followed by two police officers with guns drawn as he goes around the front of a grey SUV.

As he opens the door and tries to get into the driver's seat one of the officers pulls on his T-shirt and he appears to be shot repeatedly in the back.

Civil rights lawyer Ben Crump said Mr Blake had been trying to break up a fight between two women as his three sons watched on.

"They saw a cop shoot their father. They will be traumatised forever," Mr Crump said on Twitter.

Mr Crump represents the family of George Floyd, a Black man who died on May 25 when a white police officer knelt on his neck for nearly nine minutes.

The shooting sparked outrage in Kenosha, which is located nearly 65 kilometres south of Milwaukee, as protesters gathered at the scene of the shooting. Several businesses in the area were vandalised and about 50 cars were set on fire at a local car lot, local news station WDJT reported.

"I know he's got kids and a girl. I never had a problem with him. I always see him every day. [He] parks right behind me on the street," one neighbour told the outlet.

After nightfall in Kenosha, a crowd of protesters faced off against riot police, according to footage posted by the Milwaukee Journal Sentinel.

Kenosha city later declared an overnight curfew.

Wisconsin Department of Justice said its criminal investigation division was investigating the shooting.

"The involved officers have been placed on administrative leave," it said in a statement.

The death of Floyd ignited massive global protests against racism and police brutality.

Originally published as Horrifying moment another black man is shot by US cops