US President Barack Obama called the police shooting death of an unarmed black teenager a tragedy on Tuesday and urged a thoughtful response after two nights of violent protests, looting and arrests in the northern St. Louis suburb where the shooting took place.

He promised a full investigation by the US Justice Department into the case, which has provoked outrage in the largely African-American city of Ferguson.

"I know the events of the past few days have prompted strong passions, but ... I urge everyone in Ferguson, Missouri, and across the country, to remember this young man through reflection and understanding," Obama said.

Friends and family of 18-year-old Michael Brown held a peaceful church vigil on Tuesday night, and his father pleaded for an end to the violence. Standing with supporters, including the Rev. Al Sharpton, Michael Brown Sr. said he wanted justice for his son but wanted it "the right way."

"I need all of us to come together and do this right, the right way," said the elder Brown, who wore a T-shirt showing his son's baby picture. "No violence."

Several hundred protesters appeared to heed the calls for nonviolence late on Tuesday evening, chanting "hands up, don't shoot" and "no justice, no peace" during a tense but ultimately peaceful standoff with police clad in riot gear and flanked by armored vehicles near the site of Brown's death.

50 arrested

The protesters, some of whom waved signs as the group was led in chants by megaphone, had dwindled to a handful before midnight.

Sharpton, a New York-based civil rights leader, called for peaceful protest in the wake of looting and over 50 arrests since the shooting. Sharpton's National Action Network will pay for Brown's funeral.

"To become violent in Michael Brown's name is to betray the gentle giant that he was," Sharpton said of the 198-cm Brown, who had planned to start college this week.

The activists also were demanding authorities make public the name of the officer involved. The police had said they would release the officer's name on Tuesday, but changed the plan, citing fears of retaliation, according to media reports.

Police said Brown was shot in a struggle with a gun in a police car but have not said why Brown was in the car. At least one shot was fired during the struggle and then the officer fired more shots before leaving the car, police said.

Conflicting accounts

The FBI has opened a civil rights investigation into the racially charged case, and St. Louis County also is investigating.

A witness to the shooting interviewed on local media has said that Brown had been putting his hands up to surrender when he was killed.

"There were many, many witnesses who have talked to family members, and they paint a very different picture than police witnesses," said Benjamin Crump, an attorney for the Brown family. Crump also represented the family of Trayvon Martin, an unarmed black teen killed in Florida by a neighborhood watch volunteer in 2012.

The "hands up" gesture has been frequently seen at protests over the shooting. More than 100 protesters in front of the St. Louis County Courthouse in nearby Clayton on Tuesday morning chanted "hands up, don't shoot".

Demonstrations on Sunday night turned violent, with looting and property damage. Violence broke out again on Monday night as police officers in riot gear, armed with rifles and accompanied by dogs, tried to secure the area.