Clashes between demonstrators and riot-clad police have marred the end of a second day of protest rallies in St Louis following the acquittal of a white police officer in the fatal shooting of a black man.

Several hundred people gathered near Washington University after dark as the main rallies and marches wound down on Saturday night.

As the night wore on violence broke out between some of roughly 100 remaining protesters, some holding bats or hammers, and riot-clad police who ordered them to disperse.

'It is our duty to fight for our freedom,' shouts LaShell Eikerenkoetter. Photo / AP
'It is our duty to fight for our freedom,' shouts LaShell Eikerenkoetter. Photo / AP

Shop and restaurant windows were smashed, including at businesses crowded with patrons, and demonstrators hurled rubbish cans at officers, who numbered about 200.

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Police declared the scene an unlawful assembly and threatened to deploy tear gas and arrested at least eight people.

"We had been getting such a good turnout earlier and it was a peaceful protest," said Jomar Jackson, 32. "But then a bunch of people came and decided to be disruptive."

The demonstrations began peacefully on Friday after Circuit Judge Timothy Wilson acquitted former St. Louis Police Officer Jason Stockley, 36, of first-degree murder in the shooting death of Anthony Lamar Smith, 24.

But as on Saturday, Friday's protests ended in late night violence, with 33 arrests after clashes on which 10 officers were hurt, said news.com.au.

"We don't want to see property destruction or see people getting hurt," Elad Gross, a 29-year-old St. Louis civil rights lawyer, told Reuters as activists gathered peacefully at another protest site in a park on Saturday.

"But this is a protest that addresses injustices not only happening here in St. Louis but around the country."

Rock band U2 cancelled a concert scheduled for Saturday in St. Louis, and singer Ed Sheeran did the same for his show on Sunday, citing security concerns.

A man yells at police in riot gear just before a crowd turned violent. Photo / AP
A man yells at police in riot gear just before a crowd turned violent. Photo / AP

The verdict came about three years after rioting broke out in the St. Louis suburb of Ferguson when a black teenager was shot dead by a white police officer.

After Friday's ruling, some 600 chanting protesters marched from the courthouse through downtown St. Louis, some of them holding "Black Lives Matter" signs.

Later, some broke windows at a library and two restaurants and threw bricks and bottles at officers, who used tear gas and rubber bullets to disperse the throng. At one point, demonstrators threw rocks and paint at the home of St. Louis Mayor Lyda Krewson, police said.