Authorities in Missouri must respect protesters' right to peacefully demonstrate following the Grand Jury's ruling over the Michael Brown shooting case, NGOs have said.

The comments came after officer Darren Wilson, the white policeman who shot dead the 18-year-old African-American boy in Ferguson on 9 August, was acquitted.

An audio recording examined by the FBI alleged that Wilson had fired 11 shots. Brown, who was unarmed, was hit by six bullets, two of which penetrated his skull.

The shooting sparked world-wide outrage, and as the acquittal has sparked further protests, NGOs are warning authorities not to use violence against demonstrators.

"The right to peaceful demonstration is a human right that must be protected vigilantly," said Amnesty International USA Executive Director Steven W Hawkins. "Officers are duty-bound to respect and facilitate that right, not impede it.

"People must be assured that measures will be taken to prevent unnecessary or excessive force. The actions of law enforcement in the next few days will be absolutely critical to provide the necessary confidence that lessons have been learned. Amnesty International, and indeed the world, will be watching."

Human Rights Watch said: "Brown's killing touched off large public demonstrations, beginning the evening of August 10 and continuing mostly unabated for two weeks.

"Human Rights Watch found serious problems in the police response to the protests, including the intimidation of protesters that infringed upon their rights to assembly and freedom of expression. At times, the police used unnecessary and disproportionate force.

"In the event of renewed protests, police should permit peaceful assembly and expression, refrain from using excessive force, and conduct operations with transparency and accountability."

However, the comments came as protesters in Ferguson clashed with police and set alight buildings and cars, with up to four people being shot.

Brown's family issued a statement expressing sadness, but calling for peaceful protest and a campaign to require body cameras on police officers nationwide.

"We are profoundly disappointed that the killer of our child will not face the consequence of his actions," the statement said. "While we understand that many others share our pain, we ask that you channel your frustration in ways that will make a positive change. We need to work together to fix the system that allowed this to happen."

Following the verdict hundreds of people are also taking to social media to voice their dissent.

Twitter users are using the hashtags #HandsUpDontShoot and #FergusonDecision to comment on the issue.