Media playback is unsupported on your device Media caption As Aleem Maqbool reports, a second man was shot on Tuesday after brandishing a knife at police officers

US police say they have arrested 31 people during another night of angry protests in the town of Ferguson in the state of Missouri.

Unrest flared hours after President Obama called for calm following the fatal police shooting of unarmed black teenager Michael Brown on 9 August.

The US attorney general is due to visit Ferguson on Wednesday to meet federal officials investigating the killing.

The National Guard has been deployed to support police operations.

On Tuesday, police shot and killed a knife-wielding man four miles (6.4km) from the site of Mr Brown's killing.

Local police said witnesses reported the man behaving erratically, and that the man had come at two officers with the knife.

"The suspect turned toward officers, pulled out knife in an overhand grip, told officers to 'shoot me now, kill me now,'" said St Louis Police Chief Sam Dotson.

Missouri Governor Jay Nixon announced the National Guard deployment on Monday, in addition to lifting a night-time curfew imposed on the weekend.

The killing of Mr Brown by a white policeman in a street has inflamed racial tensions in the largely black community of Ferguson.

Media playback is unsupported on your device Media caption A new poll by the Pew Research Center has found US reaction to the police killing of Michael Brown falls along distinct racial lines.

Officer Darren Wilson shot the teenager last week after reportedly stopping him for walking in the street.

'Violent agitators'

Police Capt Ron Johnson said officers were forced to use tear gas and stun grenades on Tuesday after they came under "heavy gunfire" and were attacked with petrol bombs and bottles.

He said two people were hit by gunfire from demonstrators, insisting that officers did not use their weapons.

Image copyright Getty Images Image caption Police and protesters clashed again, 10 days after an officer shot dead an unarmed black teenager

Image copyright PA Image caption Officers made several arrests on Monday night as the violence escalated

Image copyright AP Image caption A protester had milk poured over his face to counter the effects of tear gas

Image copyright Getty Images Image caption The killing has exposed racial tensions in the majority-black suburb of St Louis

Capt Johnson appealed to demonstrators to protest in the daytime to prevent a "tiny minority of law breakers" from deliberately sparking violence.

"Anyone who has been at these protests understands that there is a dangerous dynamic in the night: it allows a small number of violent agitators to hide in the crowd and then attempt to create chaos," he told journalists.

Media playback is unsupported on your device Media caption Local resident: "When they started throwing tear gas, I started running"

Those arrested include people from other states, including New York and California, Capt Johnson said. Correspondents say this suggests the protests are becoming more organised.

In an earlier statement, US Attorney General Eric Holder said he would "personally travel" to Ferguson on Wednesday to meet FBI investigators and prosecutors.

"I realise there is tremendous interest in the facts of the incident that led to Michael Brown's death, but I ask for the public patience as we conduct this investigation," Mr Holder said.

He added that the investigation into the shooting of 18-year-old Mr Brown was "a critical step in restoring trust between law enforcement and the community, not just in Ferguson, but beyond".

Media playback is unsupported on your device Media caption "A small number of violent agitators... hide in the crowd and then attempt to create chaos" - Police Captain Ron Johnson

Media playback is unsupported on your device Media caption "The majority of people were quite peaceful" - Prof Stefan Bradley, St Louis University

Separately, President Barack Obama said he understood the "passions and anger" provoked by the death of the teenager.

But he also stressed that giving into anger "by looting or carrying guns and even attacking the police only serves to raise tensions and stir chaos".

Mr Obama said he recognised that in many communities in the US a "gulf of mistrust" existed between local residents and law enforcement.

"In too many communities, too many young men of colour are left behind and seen only as objects of fear," he said.

Ferguson unrest 21,205 Population of Ferguson, Missouri 65% Black or black African 6% Police officers are black

9% Unemployed

21% Families living below the poverty line Getty

Image copyright Getty Images Image caption Pathologist Dr Michael Baden indicates where a gunshot struck Michael Brown

Earlier, a former New York pathologist hired by Mr Brown's family performed an independent post-mortem examination.

Dr Michael Baden said on Monday he believed six bullets struck the teenager, two in the head.

"All of the gunshot wounds could have been survivable, except the one at the top of the head," he said.

Dr Baden said there were no signs of a struggle, as abrasions around the teenager's face were likely from falling to the pavement after being shot.

He also believed Mr Wilson did not shoot him at close range as there was no gunpowder residue on his body, suggesting the officer was more than 2ft (60cm) away.

Media playback is unsupported on your device Media caption Family lawyer Benjamin Crump said autopsy results backed up the witness accounts

Witnesses have said Mr Brown was shot as he held his hands up in a position of surrender, while the police and supporters of Mr Wilson have said he fired during a fight with Mr Brown.

The officer who shot Mr Brown, Darren Wilson, has been suspended with pay since the shooting. Mr Brown's family have called for his arrest.

A third post-mortem on Mr Brown was conducted by the US justice department on Tuesday, in addition to examinations by Dr Baden and St Louis county officials.

Timeline of events

Image copyright Other - Facebook Image caption Michael Brown had recently graduated from high school.