The US city of St Louis saw its third consecutive night of protest on Sunday as crowds demonstrated against the acquittal of a former policeman in a first-degree murder case relating to the 2011 shooting death of Anthony Lamar Smith.

Demonstrators lay down in front of police headquarters, simulating death in a "die-in" as police watched on from the windows of the building. They then resumed large-scale marching through the streets, similar to previous days, chanting "stop killing us."

Read more: St. Louis: Protests after white officer acquitted in fatal shooting of black man

But more chaotic scenes happened after organizers announced the end of the protest, which had gone on peacefully for hours.

Protesters smashed windows and tried to block an interstate highway ramp, police and witnesses said.

"Once again, a group of criminals set out to break windows and destroy property," St Louis police's Larry O'Toole told reporters late on Sunday local time. "Tonight, those criminals are in jail. Tonight we made more than 80 arrests. We confiscated at least five weapons; none of them were used. Some criminals insulted law enforcement officers or threw chemicals or rocks at them. All the officers' injuries were minor or moderate; they will return to duty soon." 

"Today we saw again that the vast majority of protesters are non-violent, but, for a third day in a row, the days have been calm and the nights have been destructive," Mayor Lyda Krewsen said at the same press briefing. "After the demonstrations, organizers announced that the daytime protest was over, but a group of agitators stayed behind, apparently intent on breaking windows and destroying property. This is not acceptable."

Police in the city in the central state of Missouri donned riot gear as they ordered protesters to disperse and detained several people including news photographers.

"This is no longer a peaceful protest," St. Louis police said on Twitter, adding that one officer was injured.

"Police continue to make arrests after property damage in Downtown #STL," another tweet said.

The scenes were reminiscent of the more serious clashes that broke out in 2014 in the St Louis suburb of Ferguson after the killing of 18-year-old Michael Brown by a white police officer, who was not indicted over the shooting.

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A few troublemakers

Organizers said they were frustrated that a few troublemakers could tarnish their non-violent message.

Protesters were angered by the not-guilty verdict against Jason Stockley, who shot Smith after being led on a high-speed chase as he and his partner tried to arrest him for a suspected drug deal.

Stockley told the court that he felt he was in danger because he saw Smith holding a silver revolver.

But prosecutors accused Stockley of planting the gun, given the officer's DNA was on the weapon but Smith's wasn't. Dashcam video recorded Stockley saying he was "going to kill this (expletive)." Less than a minute later, he shot Smith five times.

aw/msh (AP, Reuters)