An attacker with an automatic weapon opened fire on police on Paris' iconic Champs-Elysees, killing one officer and seriously wounding two others before police shot and killed him.
The Islamic State (IS) group quickly claimed responsibility for the attack.
Paris police spokeswoman Johanna Primevert told The Associated Press that the attacker on Thursday targeted officers guarding the area near the Franklin Roosevelt subway station at the center of the sparkling shopping boulevard popular with tourists. She said he appeared to be acting alone.
Police and soldiers sealed off the area, ordering tourists back into hotels and blocking people from approaching the scene.
The attack came three days before the first round of balloting in France's tense presidential election. Security has been especially high since Tuesday, when police said they thwarted a terror attack by arresting two men.
The Paris prosecutor's office said counterterrorism investigators are involved in the probe of yesterday's attack. Two police officials, speaking on condition of anonymity because they weren't authorised to publicly discuss the investigation, said the attacker had been flagged as an extremist. They had no other details about him.
The claim of responsibility several hours after the attack came unusually swiftly for the IS group, which has been losing territory in Iraq and Syria.
In a statement from its Amaq news agency, the group gave a pseudonym for the shooter, Abu Yusuf al-Beljiki, indicating he was Belgian. Soon after the Arabic-language statement, the group sent one out in French, describing it as an attack "in the heart of Paris."
Interior Ministry spokesman Pierre-Henry Brandet said on BFM television that a man came out of a car and opened fire on a police vehicle. He said officers were "deliberately" targeted.
Speaking in Washington during a news conference with Italian Prime Minister Paolo Gentiloni, US President Donald Trump said the shooting "looks like another terrorist attack" and sent condolences to France.
Emergency vehicles blocked the wide avenue lined with shops that cuts across central Paris between the Arc de Triomphe and the Tuileries Gardens, normally packed with cars and tourists. Subway stations were closed off.