St Louis County has declared a state of emergency after a gunman fired at police during a day of protests.

US police officers shot and critically wounded a man who opened fire at them, ending what had been a day of remembrance for an unarmed black teen killed by police a year earlier.

After a peaceful protest on the streets of the St Louis suburb of Ferguson to mark the fatal shooting of 18-year-old Michael Brown, several shots were fired during clashes in the city late on Sunday.

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The shooting of Brown by a white police officer 12 months ago triggered violent unrest and highlighted racial tensions in America.

In a statement on Monday, St Louis County executive Steve Stenger said county police would immediately take charge of “police emergency management” in Ferguson and surrounding districts.

Protesters who had gathered in the street reportedly fled in multiple directions, hiding behind cars and taking cover in buildings.

St Louis County Police Department tweeted that an officer was involved in a shooting after coming under heavy gunfire.

“During the gunfire, at least two unmarked cars took shots,” it added.

Video footage shows a black man lying facedown on the ground in handcuffs, bleeding profusely.

St Louis County Police Chief Jon Belmar said there was “a remarkable amount of gunfire”, with as many as 50 bullets let loose in the initial shooting between the two groups.

The detectives involved had from six to 12 years of experience and were placed on administrative leave.

Multiple shots fired in Ferguson. Please leave the area of Ferguson and West Florissant.

— St. Louis County PD (@stlcountypd) August 10, 2015

(1/2) A St. Louis County officer was involved in an officer-involved shooting after comin g under heavy gunfire. pic.twitter.com/L4PqopaEF1

— St. Louis County PD (@stlcountypd) August 10, 2015

Belmar said those who had started the shooting were not protesters but “criminals”.

“There is a small group of people out there that are intent on making sure that we don’t have peace that prevails,” he said.

A crowd of about 300 people had gathered earlier to mark the anniversary, during which they observed four-and-a-half minutes of silence and released two white doves.

The time represented the four-and-a-half hours that Mr Brown’s body lay face down in the street before being taken away.

Many in the crowd wore T-shirts emblazoned with Mr Brown’s portrait and the words “Choose Change”.

Others carried signs, including one that read: “STOP killing black children”.

Mr Brown’s father, also called Michael, said he was grateful so many people had turned out for the march.

“If it wasn’t for y’all this would be swept under the carpet. So I just want to give my love out to y’all,” he said to the crowd.

In New York, dozens of people gathered at Union Square to hold a vigil for Mr Brown in solidarity with Ferguson and to call for ongoing demonstrations against police killings of minorities.

About 100 people gathered in Brooklyn earlier, staging a symbolic “die-in” to protest Mr Brown’s shooting.

Police arrested several people.

One year on, black leaders say they have witnessed a dramatic change in American attitudes toward race but see little being done to enact reforms.

The head of the National Association for the Advancement of Colored People, one of the country’s oldest civil rights group, called the pace of legislative change “glacial”.

– with AAP