A police officer has been killed and two others injured in a suspected terrorist shooting in the centre of Paris.
The attacker emerged from a car and used an automatic weapon to shoot at officers outside a Marks & Spencer store at the centre of the Champs-Elysees, anti-terrorism prosecutor Francois Molins said.
The gunman was shot dead by other officers, and a woman tourist was also wounded in the incident on Thursday night.
Police have launched a terrorism investigation and the so-called Islamic State extremist group claimed responsibility for the attack.
French officials said the suspect was previously flagged as an extremist.
The world-famous avenue was sealed off and nearby stations on the Paris Metro closed as investigations began.
French police warned the public to stay away from the area and armed officers were at the scene.
They also asked people not to "spread any misinformation" that has not come from a "trusted source".
Tourists were warned to go back to their hotels and to let family and friends know they were safe.
The UK Foreign Office said: "The British embassy is in contact with local authorities and urgently seeking further information following reports of a shooting incident on the Champs-Elysees in Paris.
"You should remain vigilant and follow the advice of the local security authorities and/or your tour operator.
"If you're in the area and it is safe to do so, contact your friends and family to tell them you are safe."
A Downing Street spokesman said: "The UK strongly condemns the appalling terrorist attack in Paris.
"The Prime Minister has tonight passed on her condolences to President Hollande."
The incident comes days before the French election, which is to take place on Sunday.
President Francois Hollande called an emergency meeting on Thursday evening and said at a press conference that he was convinced the attack was a terrorist act.
US president Donald Trump, speaking at a press conference in Washington, said the incident "looks like another terrorist attack" and sent his condolences to France.
Steve White, chairman of the Police Federation of England and Wales, said: "Once again the police family is in mourning and it serves to underline how brave officers are putting their life on the line day in and day out to protect the public.
"As French colleagues felt the pain of our loss in Westminster last month, we feel theirs tonight."
Pictures of the scene in Paris showed people rushing to clear the area, and a heavy armed police presence.
Eyewitness Badi Ftaiti said: "They were running, running.
"Some were crying. There were tens, maybe even hundreds of them."
After the shooting, police swooped on an address in a Paris suburb.
A police document obtained by The Associated Press identifies the address searched in the town of Chelles as the family home of 39-year-old convicted criminal Karim Cheurfi.
Archive reports by French newspaper Le Parisien say he was convicted of attacking a police officer in 2001.