Julia Jackson (center), mother of Jacob Blake, speaks next to National Civil Rights Attorney Ben Crump (right) during a press conference in Kenosha, Wisconsin on Tuesday. Photo: AFP

The mother of a black man shot repeatedly in the back by Wisconsin police called Tuesday for calm after two nights of violent protests, as her lawyer said it would take a "miracle" for her son to walk again.

"We really just need prayers," Julia Jackson said of her son Jacob Blake, seen in video being shot up to seven times at point-blank range by a white Kenosha Wisconsin police officer as he tried to get into his car with his three children inside on Sunday.

With hundreds of people marching again Tuesday - and at least one small group of mainly white, heavily armed men vowing to protect property - her call for healing took on greater urgency.

The most recent police shooting of an African-American has sparked outrage and protests in US cities including New York and Minneapolis.

The rallies have been largely peaceful though some protesters burned cars and buildings on Sunday and Monday nights in the northern city. 

Jackson was visibly emotional as she urged peace ahead of a national protest against police brutality planned for Washington this weekend, where Blake's shooting, the killing of George Floyd, and other deaths have mobilized the Black Lives Matter movement.

The damage caused on previous nights "doesn't reflect my son or my family," she said. Then she issued a powerful call for unity and denunciation of racism in America.

"Clearly you can see by now that I have beautiful brown skin. But take a look at your hand. And whatever shade it is, it is beautiful as well," she said.

AFP