Police have released footage they say shows a teenager pulling a gun from his waistband moments before he was shot by officers in Ferguson, Missouri.

Tyrone Harris Jr was critically injured when he was shot on Sunday night as demonstrators marked the one-year anniversary of Michael Brown's death.

Harris allegedly opened fire on an unmarked police van and a gun battle with police ensued.

The 18-year-old remains in a critical condition in hospital.

He has been charged with 10 felonies - five counts of armed criminal action, four counts of first-degree assault on a law enforcement officer and a firearms charge.

St Louis County Police say the 13-second video from a security camera shows Harris grabbing the gun in response to shots being fired during the protests taking place nearby.

Police Chief Jon Belmar has said the officers who shot Harris suspected he was armed and had been keeping an eye on him during the protests.

At one point, Harris allegedly approached the police vehicle and opened fire, Chief Belmar said. The officers then returned fire and pursued the suspect on foot.

The St Louis County Police Department tweeted pictures of at least two unmarked cars that were hit by gunfire.

Video captured on a mobile phone showed officers standing over the injured suspect as an onlooker can be heard urging police to "please get him some help".

No officers were injured, and those involved in the shooting have been placed on administrative leave.

Harris' father, Tyrone Harris Sr, told the AP news agency on Monday the police version of events was "a bunch of lies".

He said his son had been drawn into a dispute involving two groups of young people and was unarmed when the officers shot him eight to 12 times.

"My son was running to the police to ask for help, and he was shot," Mr Harris said.

"It's all a bunch of lies ... They're making my son look like a criminal."

He told local media that his son was "real close" to Michael Brown, who was fatally shot by Ferguson officer Darren Wilson on 9 August last year.

The town was the focus of months of massive protests following Mr Brown's killing.

Mr Wilson was cleared of wrongdoing by the US Department of Justice and a St Louis County grand jury.

A state of emergency was declared in Ferguson on Monday after clashes between police and protesters marking a year since Mr Brown was shot dead.