MISSOURI Governor Jay Nixon clamped down on demonstrations and ordered the National Guard onto the streets of the St Louis suburb of Ferguson early today.

Just hours earlier, police had again used tear gas to clear protesters off the streets following a demonstration against the fatal police shooting of black teenager Michael Brown.

Mr Nixon said the troops would “help restore peace and order” to Ferguson, which has been filled nightly with angry crowds.

The latest confrontation came on the same day that Attorney General Eric Holder ordered a federal medical examiner to perform a second autopsy on Mr Brown.

A preliminary private autopsy had found that he had been shot at least six times, including twice in the head.

There had been an angry protest on Saturday night, running into Sunday morning, and the night following was to be no different.

As the sun set a peaceful protest quickly deteriorated after police pushed the demonstrators back by repeatedly firing tear gas and the streets were empty well before the curfew took effect at midnight.

Authorities claimed that state troopers had responded to reports of gunfire, looting, vandalism and protesters who hurled petrol bombs.

Officers in riot gear ordered the protesters to disperse.

Many of the marchers retreated but a group of about 100 stood defiantly about two streets away until getting hit by another volley of tear gas.

Protesters laid a line of cinder blocks across the street near a corner shop that was burned down last week.

It was apparently an attempt to block police, but their vehicles easily ploughed through and the crackle of gunfire could be heard from several streets away.

“Based on the conditions, I had no alternative but to elevate the level of response,” said Captain Ron Johnson of the Missouri Highway Patrol, who is in command in Ferguson.

At least two people had been wounded in shootings, he said.

The US Justice Department has deepened its civil rights investigation into the shooting.

A day earlier, officials said that 40 FBI agents were going door-to-door gathering information.