An image of a gun-wielding off-duty Turkish policeman standing over the body of Russia's ambassador, whom he had just fatally shot, has won the 2017 World Press Photo competition.

Jurors and colleagues lauded the courage and composure of Associated Press photographer Burhan Ozbilici, whose image was part of a series titled An Assassination in Turkey that also won the spot news - stories category.

The photos were captured in the moments before and after policeman Mevlut Mert Altintas drew a handgun and shot ambassador Andrei Karlov at a photo exhibition in Ankara on December 19.

In the winning photo, the gunman, wearing a suit and tie, stands defiantly, gun in his right hand pointed at the ground and with his left hand raised, his index finger pointing upward.

His mouth is wide open as he shouts angrily.

The ambassador's body lies on the floor just behind Altintas.

Another image in the series showed the ambassador before the shooting, with Altintas standing behind him.

Mr Ozbilici said his professional instincts kicked in despite the shocking scene unfolding in front of him.

"It was extremely hot, like I had boiled water on my head, then very cold, very cold. Extremely dangerous," Mr Ozbilici said in an interview.

"But at the same time I understood that this was big history, it was history, (a) very, very important incident."

So the veteran AP photographer did what he has learned to do over some 30 years.

"I immediately decided to do my job because I could be wounded, maybe die, but at least I have to represent good journalism," he said.

The winning image was among 80,408 photos submitted to the prestigious competition by 5,034 photographers from 125 countries.

The jury awarded prizes in eight categories to 45 photographers from 25 countries.

"Burhan's striking image was the result of skill and experience, composure under extreme pressure and the dedication and sense of mission that mark AP journalists worldwide," said AP executive editor Sally Buzbee.

"We are enormously proud of his accomplishment."

Jury chairman Stuart Franklin called Mr Ozbilici's image "an incredibly hard-hitting news photograph" and part of a strong series documenting the assassination.

"I think Burhan was incredibly courageous and had extraordinary composure in being able to sort of calm himself down in the middle of the affray and take the commanding pictures that he took," Mr Franklin said.

"I think as a spot news story it was terrific."

Denis Paquin, AP's acting director of photography, said Mr Ozbilici's actions that day were typical of his professionalism.

"Burhan would tell you he was just doing his job. His humble professionalism, combined with incredible courage, enabled him to capture these unforgettable images," he said.

Speaking after receiving the award, Mr Ozbicili said he felt a responsibility to his entire profession as he stood his ground in the Ankara gallery.

He said he felt like journalists from all over the world were saying: "Hey man, you are representing all of us. Don't go away! Stand! We are supporting you, we are praying for you."

Mr Ozbicili said the award would not change him, "but I will have more responsibility. The young generation, especially, they need more support, more courage to continue because their journalism, independent journalism, is suffering. So I wanted to show with this photo that the world will need good journalism forever".

The eclectic selection of winners highlighted the dominant news topics of the last year - including conflict in Syria and Iraq, the migrant crisis, the death of longtime Cuban leader Fidel Castro and the Olympic Games in Rio.

Among winning nature photos were images depicting humanity's devastating effect on wildlife, including a gruesome photograph of a poached rhino with its horn hacked off and another showing a turtle swimming while enmeshed in a green fishing net.

Among other winners, Jonathan Bachman of the United States, a photographer for Thomson Reuters, won the contemporary issues - singles category with an image of Ieshia Evans being detained in Baton Rouge during a protest on July 9 over the death of Alton Sterling, a black man killed by police.

Ms Evans stands bolt upright in a flowing dress as two police officers in heavy body armour and helmets move to take her into custody.

Mr Franklin called Mr Bachman's image "an unforgettable sort of comment on passive resistance. It's really a lovely photograph. You'll never forget it".

For the first time, World Press Photo awards for still images were announced at the same time as those for digital storytelling, with prizes awarded for innovative storytelling, immersive storytelling, long form and short form covering issues including modern relationships, the rise of walls and fences around the world and the story of an American boxer from Flint, Michigan.

Among media whose work was recognised were The New York Times, The Washington Post and smaller independent producers.