Hundreds of protesters gathered at a suburban St Louis police station today, demanding murder charges against an officer who shot dead an unarmed black teenager over the weekend.

The largely peaceful protests, monitored by about 50 police officers in riot gear, took place after a night of rioting when demonstrations over the death of Michael Brown,18, turned violent.

About two dozen businesses were damaged in the rioting overnight, 32 people were arrested, and two officers were injured, police said.

"It breaks my heart," Ferguson Police Chief Tom Jackson said.

"Last night was the worst night of my life," he added.

The FBI is looking into any civil rights violations in the case, a spokeswoman for the agency said.

St Louis County police is leading the investigation into the shooting itself.

Mr Jackson said there was plenty of physical evidence and eyewitness testimony in the case. "I really believe we can get to the truth of what happened here," he said.

Mr Brown's family has asked for an end to the street violence, according to a statement obtained by KTVI television station.

They have also hired lawyer Benjamin Crump, who represented the family of Trayvon Martin, a black Florida teenager who was also unarmed when he was shot in 2012, Mr Crump's office said.

Today's gathering appeared non violent, with protesters shouting "Stop the killer cops" and "No justice, no peace." Four people were arrested, according to KSDK-TV.

Police said Mr Brown was shot on Saturday afternoon after a struggle over a gun in a police car.

It was not immediately clear why Mr Brown was in the car.