Protests in a US suburb rocked by racial unrest after a white police officer killed an unarmed black teenager has turned violent.

Molotov cocktails were hurled at police who responded with smoke bombs and tear gas to disperse the crowd in Ferguson, Missouri.

Police chief Thomas Jackson, who has been the public face of the city torn by the death on Saturday of Michael Brown, 18, said earlier that the St Louis County investigation of the shooting could take weeks to complete.

In the meantime, he said, his department welcomed Justice Department training on race relations in the St Louis suburb, where two thirds of the 21,000 residents are black, while all but three of the police force's 53 officers are white.

"Unfortunately, an undertow (of racial unrest) has bubbled to the surface," said Mr Jackson. "Race relations is the top priority right now."

While Mr Jackson said he wanted to mend fences with the community, protesters were on the streets of Ferguson again overnight, facing heavily armed police who at times trained weapons on them from an armoured vehicle.

Two reporters said they were detained by police while working at a McDonald's restaurant in the area and the situation became more tense after nightfall, with police ordering people to go home, then using smoke bombs and later tear gas after they said some people threw Molotov cocktails at them.

Some civil rights leaders have drawn comparisons between Mr Brown's death and that of 17-year-old Trayvon Martin, who was shot dead by a Florida neighbourhood watch volunteer who was later acquitted of murder charges.

The case has provoked a broad discussion on social media sites about the death of young black men in racially-tinged shootings. On Twitter, a campaign called IfTheyGunnedMeDown prompted many black users to post photos of themselves and ask how they might be portrayed in news reports if they became shooting victims.

Earlier, Wesley Lowery of The Washington Post and Ryan Reilly of The Huffington Post said they were handcuffed and put into a police van after officers came in to clear the McDonald's where they were doing some work.

Mr Lowery said that at one point he was slammed against a drinks machine and plastic cuffs were put on his wrists. The reporters were subsequently released without charge.

Post executive editor Martin Baron said "there was absolutely no justification for his arrest" and that the organisation was appalled by the conduct of the officers involved.

Police have said the shooting happened after an officer encountered Mr Brown and another man on the street. They say one of the men pushed the officer into his squad car, assaulted him in the vehicle and struggled with the policeman over his gun. At least one shot was fired inside the car.

The struggle then spilled on to the street, where Mr Brown was shot several times. In their initial news conference about the shooting, police did not specify whether Mr Brown was the person who scuffled with the officer in the car and have refused to clarify their account.

Mr Jackson said yesterday that the officer involved had facial injuries.

Dorian Johnson, who says he was with Mr Brown when the shooting happened, told a much different story. He said the officer ordered them out of the street, then tried to open his door so close to the men that it "ricocheted" back, apparently upsetting the officer.

Mr Johnson said the officer grabbed his friend's neck, then tried to pull him into the car before brandishing his weapon and firing. He said Mr Brown started to run and the officer pursued him, firing several times. Mr Johnson and another witness said Mr Brown was on the street with his hands raised when the officer fired at him repeatedly.

Some protesters raised their arms above their heads as they faced the police yesterday, while others held signs asking for answers about Mr Brown's death. The most popular chant has been: "Hands up! Don't shoot!"

Mr Brown's body remained on the street for hours - a span Mr Jackson deemed "uncomfortable" but justified, given that "you only get one chance at that crime scene" to process it correctly. He said authorities were also concerned about gunfire they could hear in a nearby building.

In the shooting's aftermath, notorious hacking collective Anonymous said it burrowed into the city website and shut it down for much of Monday. The group also released what it said were audio experts from St Louis County dispatch on the day Mr Brown was killed. Police declined to comment on the recordings.