Police began firing tear gas and stun grenades at demonstrators in Ferguson, Missouri, Monday night following days of unrest sparked by the fatal shooting of an unarmed black teenager by a white policeman.

Witnesses said that the action by police came after hours of a mostly peaceful but tense street protest.

The latest clashes came at the end of a day in which a pathologist hired by the Brown family said the unarmed 18-year-old suffered a bullet wound to his right arm that may indicate his hands were up or his back was turned. But the pathologist said the team that examined Brown cannot be sure yet exactly how the wounds were inflicted until they have more information.

Protesters filled the streets after nightfall, and officers used bullhorns to order them to disperse. Police deployed noisemakers and armoured vehicles to push demonstrators back. During one confrontation, officers fired tear gas and flash grenades. There were no reports of serious injuries.

After the streets had been mostly cleared, authorities ordered reporters to leave as well, citing the risk from gunfire that had been reported.

As darkness fell Monday National Guard units with armoured vehicles were waiting at a staging area about a half-mile from the portion of West Florissant Avenue that has been the scene of the largest protests since the fatal police shooting of Michael Brown nine days ago.

As a crowd of demonstrators was marching and growing in size, sheriff's deputies in body armour and state troopers carrying wooden bats and gas masks stood watch over the group.

Police were telling protesters Monday that they could not assemble in a single spot, saying they had to keep moving. In federal court, a judge denied a request from the American Civil Liberties Union for a restraining order that would have prevented authorities from enforcing the rule.

Two men were arrested for disorderly conduct and failure to disperse, police said. Scott Olson, a photographer for the Getty photo agency covering the events since Aug. 11, was arrested while covering the demonstrations and later released.

St. Louis native Nelly, centre, walks the streets with protesters Monday in Ferguson. (Jeff Roberson/The Associated Press)

Authorities were also establishing a designated protest zone for nightly demonstrations. The plan was announced Monday by St. Louis County police.

It was not clear what would happen to those who refuse to use the area along West Florissant Avenue, where the majority of protests have occurred.

A grand jury could begin hearing evidence Wednesday to determine whether the officer, Darren Wilson, should be charged in the death of the 18-year-old Brown, said Ed Magee, spokesman for St. Louis County's prosecuting attorney.

Obama calls for peace

Earlier Monday, U.S. President Barack Obama urged "rightly hurting" protesters to remain peaceful, and called on police to do the same.

Getty Images staff photographer Scott Olson was arrested on Monday while covering the protests in Ferguson, Missouri. (Joe Raedle/Getty) The National Guard was called in by Missouri Gov. Jay Nixon after another night of violent clashes on Sunday. Nixon removed, however, a midnight curfew that had been in place on Saturday and Sunday.

St. Louis police confirmed via Twitter that protesters were arrested without incident on Monday for failure to disperse. The police department said the first of the day’s protests, a march in Kiener Plaza was peaceful.

"We are committed to ensuring every citizen’s constitutional rights of freedom of speech and assembly in a peaceful and lawful manner," the department wrote on Twitter Monday evening.

At his White House press conference, Obama said the vast majority of protesters in Ferguson were peaceful, but warned that a small minority was undermining justice.

Obama expressed sympathy for the "passions and anger," but said giving in to that anger through looting and attacks on police only stirs tensions and leads to further chaos. He said overcoming the mistrust endemic between many communities and their local police would require Americans to "listen and not just shout."

U.S. President Barack Obama urged protesters in Ferguson to remain peaceful. (Charles Dharapak/Associated Press)

"That's how we're going to move forward together, by trying to unite each other and understand each other and not simply divide ourselves from one another," Obama said.