A police officer has died and two are seriously wounded after a shooting near the Champs-Elysees boulevard in Paris.
Isis have claimed responsibility for the attack, in which the officer was said to have been killed while in a car before the assailant was himself shot dead by police.
French President Francois Holland said he was convinced the "cowardly killing" on the busy shopping street in the centre of the capital was an act of terrorism.
Paris prosecutors said they had identified the suspect, but would not release his name until it had been assessed whether he acted alone. Officials said the suspect had previously been flagged as an extremist.
The home of the dead attacker in an eastern suburb of Paris was being searched, police said.
A police source said the shooting was very probably a “terrorist act”, according to Reuters, while other reports suggested robbery may have been the motive.
The French interior ministry said it was too early to judge the reason behind the attack, but a counter-terrorism inquiry has been opened.
It came just days before French voters were set to go to the polls in the first round of the presidential election.
A witness told French television station BFM that she heard a shooting and saw a man's body on the ground, and that the area was quickly evacuated by police.
Another reportedly said they saw a man get out of a car and begin firing a Kalashnikov rifle.
Hommage au policier tué sur les Champs-Elysées ce soir, pensées à sa famille. Solidarité avec ses collègues blessés et leurs proches.— Bernard Cazeneuve (@BCazeneuve) April 20, 2017
Paris police spokeswoman Johanna Primevert said the attacker targeted police guarding the area near the Franklin Roosevelt subway station, at the centre of the avenue popular with tourists.
France has lived under a state of emergency since 2015 and has suffered a spate of Islamist militant attacks that have killed more than 230 people in the past two years.
In a statement from the Isis-affiliated Amaq news agency, the extremist group gave a pseudonym for the shooter, indicating he was Belgian.
French Prime Minister Bernard Cazeneuve tweeted that his thoughts were with the officers and their colleagues and families.
Presidential candidate Emmanuel Macron said he stood in solidarity with law enforcement workers, while Francois Fillon also tweeted a tribute to officers.
Marine le Pen and Jean-Luc Melenchon made similar statements.
“We shall be of the utmost vigilance, especially in relation to the election,” said President Hollande, who is not himself running for re-election.
Earlier this week, two men were arrested in Marseille who police said had been planning an attack ahead of the election.
US President Donald Trump said: “It looks like another terrorist attack. What can you say – it never ends. We have to be strong and be vigilant.”