US Attorney General Eric Holder has ordered a second post mortem on Missouri teenager Michael Brown who was shot dead by police on 9 August.

Mr Holder called for a federal medical examiner to carry out the autopsy in addition to the one conducted by state medical examiners "due to the extraordinary circumstances involved in this case and at the request of the Brown family."

The family is also planning to have a pathologist conduct an independent examination of the body, a family spokesman said.

Missouri Governor Jay Nixon imposed a curfew yesterday after a week of racially charged protests and looting over the shooting of the 18-year-old by Ferguson police officer Darren Wilson.

The Ferguson police department has come under strong criticism for both the shooting and its handling of its aftermath. 

Unrest flared again on Friday after police released a video showing the alleged participation by Mr Brown in a shop robbery shortly before his fatal shooting.

Police said the officer who shot Mr Brown had no idea he was a robbery suspect.

One person was shot and seven people were arrested in Ferguson early this morning.

As the curfew took effect, police used loudspeakers to tell protesters to disperse.

Officers, equipped with gas masks and full-length shields, stood among and on top of armoured vehicles.

The person who was shot is in a critical condition. He was taken hospital by bystanders before police could reach him.       

Smoke and teargas canisters used by police largely dispersed the crowd, some of whom had been chanting "no justice, no curfew, no peace", while others implored the crowd not to move forward towards police.