The police officer who fatally shot an unarmed teenager in a St Louis suburb has resigned, his lawyer said.

The resignation of Darren Wilson from the Ferguson, Missouri, police force comes nearly four months after he killed 18-year-old Michael Brown and days after the announcement that he would not face criminal charges.

The incident, which has led to months of sometimes violent protests in Ferguson, has reignited a debate over race relations and the use of police force in the United States.

Neil Bruntrager, an attorney for Mr Wilson, confirmed that the officer had submitted his resignation, a move that was long anticipated, no matter the outcome of the grand jury's deliberations.

In a letter published by the St Louis Post-Dispatch, Wilson Mr said he had been told that "my continued employment may put the residents and police officers of the City of Ferguson at risk, which is a circumstance that I cannot allow."

Mr Wilson, who said he was acting in self-defence when he shot Mr Brown, said he wanted to wait until after the grand jury's decision before he made his decision to resign, according to the letter.

After a night of arson and looting in Ferguson following the decision, demonstrations this week spread to other major US cities, with some of the protests targeting shopping centres on Friday.

Yesterday, a number of activists began a seven-day march from Ferguson to Jefferson City, the state capital.

The march, reminiscent of the civil rights marches of the 1960s, was organised by the National Association for the Advancement of Colored People (NAACP).

The NAACP is calling for a reform of police practices, a new police chief in Ferguson and a national law to prevent racial profiling by police.

More than 150 people set out on the "Journey for Justice" from the Canfield Green Apartments, near the spot where Mr Brown was shot and killed, with some marchers singing the decades-old protest song "We Shall Overcome."