US police have said they came under “heavy gunfire” and 31 people were arrested during protests in Ferguson, Missouri sparked by the shooting death of a black unarmed teenager by a white policeman.

Captain Ron Johnson of the Missouri Highway Patrol, who is in charge of security in Ferguson, said bottles and Molotov cocktails were also thrown from the crowd in the early hours of Tuesday.

“Our officers came under heavy gunfire,” in one area, he told a news conference, adding that riot police had confiscated two guns from protesters and what looked like a petrol bomb.

Earlier, police fired tear gas and stun grenades at protesters as skirmishes erupted after hours of street protests that had been tense but mostly peaceful.

National Guard troops have been deployed to help quell days of racially charged rioting and looting spurred by the fatal shooting of 18-year-old Michael Brown.

Missouri’s governor on Monday lifted the curfew for the St Louis suburb that had been in place from midnight to 5am since Saturday.

As daylight was coming to an end, police with plastic handcuffs took positions and tried to clear a main thoroughfare where protests have taken place at night, directing crowds into designated protest areas.

Video from the scene showed one journalist, with cameras slung around his neck and his hands bound behind him, being led off by police. He was identified by colleagues as Getty photographer Scott Olson.

Police said at least 31 people were arrested.

US President Barack Obama called for calm and said he could not make any judgements about situation in Ferguson until investigation is complete.

“While I understand the passions and the anger that arise over the death of Michael Brown, giving in to that anger by looting or carrying guns, and even attacking the police only serves to raise tensions and stir chaos. It undermines rather than advancing justice,” he told reporters at the White House.

‘Shot six times’

The president’s statement followed claims the black teenager had been shot at least six times.

“Six bullets struck, and two may have re-entered” Brown’s body, said Michael Baden, a former New York chief medical examiner. Baden was asked by Brown’s family and lawyers to conduct the independent examination on his remains.

One of the bullets hit the top of Brown’s head, another struck his eye, while others hit his right arm, Baden told a news conference in Ferguson.

“All of the gunshot wounds could have been survivable, except the one at the top of the head,” he said.

The family said that the bullet wound on the top of the skull suggested Brown was bowing his head to submit to the officer when he was shot.

UN chief Ban Ki Moon on Monday called on US authorities to protect the right of peaceful protests over Brown’s death in Ferguson.

“He calls on all to exercise restraint, for law enforcement officials to abide by US and international standards in dealing with demonstrators,” a UN spokesman said. Al Jazeera