Media playback is unsupported on your device Media caption Eyewitness Keith Rose: "I saw one officer covered in blood - I saw officers running to the aid of that officer"

Two US police officers have been shot in Ferguson, a Missouri town hit by riots over the killing of an unarmed black teenager last year.

One officer was shot in the face and one in the shoulder, St Louis County police chief Jon Belmar said.

Both suffered "very serious gunshot injuries" but were conscious, he said.

They were shot during a demonstration after the resignation of Ferguson's police chief, which followed a report alleging racial bias in his department.

Protesters had gathered outside Ferguson police headquarters late on Wednesday, in what was a relatively low-key demonstration.

But at about midnight at least three shots were fired as the crowd of protesters was starting to break up, Mr Belmar said.

Image copyright AP Image caption US police said the injuries of the two police officers were not life threatening

Image copyright Getty Images Image caption Demonstrators gathered outside the Ferguson police department late on Wednesday

Several witnesses said the shots had come from a hill on the other side of the street from the crowd of protesters.

In a press conference on Thursday, Mr Belmar said the shots were fired from about 125yds (114m) away, but he did not specify from which direction.

"We were very close to having what happened in New York last year," Mr Belmar said, referring to two police officers shot and killed while on duty. He said he thought it was a "miracle" that such an incident had not occurred during earlier protests in Ferguson.

He also said he would "have to imagine" that some protesters "were among the shooters". Detectives were still investigating who was responsible, and no-one had been arrested.

The St Louis County police chief said after hearing the gunshots many officers had drawn their weapons but no-one had fired.

Image copyright AP Image caption Police took cover after shots were fired shortly after midnight

Image copyright AP Image caption St Louis County's police chief said it appeared officers had been deliberately targeted

One protester, Keith Rose, said he saw an officer "covered in blood", and that other officers were carrying and dragging him, leaving a trail of blood on the ground.

Demonstrators were calling for further action to be taken over the federal report, and for more resignations in the police department, Mr Rose said.

In the hours after the policemen were shot, use of the hashtag #BlueLivesMatter spiked on Twitter, driven by self-identified supporters of gun rights and other conservative causes, as well as supporters of the police.

Police chief Thomas Jackson was the sixth Ferguson official to be fired or step down. He had initially resisted calls from protesters and some state leaders to resign.

Mr Jackson was widely criticised after the shooting of 18-year-old Michael Brown in August and the weeks of demonstrations that followed.

Ferguson - key moments

9 August , 2014: White police officer Darren Wilson shoots dead unarmed black teenager Michael Brown

, 2014: White police officer Darren Wilson shoots dead unarmed black teenager Michael Brown 15 August : Authorities identify Mr Wilson, following days of protests and clashes with police

: Authorities identify Mr Wilson, following days of protests and clashes with police 10-13 October : Activists from across US hold four days of rallies and vigils in Ferguson

: Activists from across US hold four days of rallies and vigils in Ferguson 24 November : Grand jury decides not to charge Mr Wilson, triggering further demonstrations

: Grand jury decides not to charge Mr Wilson, triggering further demonstrations 4 March : Department of Justice issues report alleging widespread racial bias in Ferguson police department

: Department of Justice issues report alleging widespread racial bias in Ferguson police department 11 March: Police chief Thomas Jackson announces his resignation

How Ferguson unrest spread

Report on Ferguson police report - key findings

Brown's killing was one of several recent cases around the US in which the deaths of black men at the hands of the police have triggered protests.

In November, a St Louis County grand jury found that white police officer Darren Wilson did not break any laws when he shot Brown.

However, Brown's shooting and the riots that followed spurred a federal investigation. It found overwhelming racial bias in the town's policing practices, though Darren Wilson was cleared of civil rights violations.

The report noted public officials regularly made tickets and other minor violations "go away" for white friends, while some black residents spent nights in jail for non-payment of fines.

Racial bias in Ferguson 93% of people arrested are African Americans, whereas only: 67% of Ferguson population is black 96% of people arrested for outstanding municipal warrants are African American

95% of "Manner of walking in roadway" charges were against black people

90% of documented force was against African Americans

30% of searches of white suspects resulted in a contraband finding - compared with 24% of black suspects GETTY

A local Democratic party leader, Patricia Bynes, said "a lot of anger" had built up in Ferguson because more action had not been taken sooner.

The names of the two officers wounded on Thursday have not been released.

The one shot in the face, under his right eye, is a 32-year-old from a police department in another St Louis suburb, Webster Groves, and the other a 41-year-old from St Louis County police department.