Socialist Benoit Hamon tweeted his "full support" to police against terrorism.
If no candidate claims a majority in the first round - which is likely - there will be a run-off election between the top two candidates on May 7.
Roads to the heart of the French capital and Arc de Triomphe have been blocked off by police, and it is recommended that people avoid the area altogether.
SHAPIRO: And as we've said, ISIS claimed responsibility for the attack.
Le Parisien newspaper, citing its own sources, reported the solo gunman fired at a police patrol with an automatic weapon.
At least one police officer was killed, two wounded and the attacker neutralised in the shooting, according to CNN affiliate BFMTV and the French Interior Ministry.
Brandet refused to give any specific detail about the suspect's possible criminal history or affiliations.
He says the assailant's identity has not been formally confirmed. "One or more perpetrators were taken down by our colleagues, there was a cross fire".
It is understood the assailant stepped out of a vehicle at a red light and opened fire with a Kalashnikov machine gun, also known as an AK-47. He says counterterrorism authorities are investigating.
The gunman is believed to have deliberately targeted the officers and was known to security services.
Another reportedly said they saw a man get out of a auto and begin firing a Kalashnikov rifle.
A look at extremist attacks in France in the past few years
A man emerged from the vehicle and opened fire on the van with an "automatic weapon", killing one officer instantly, Brandet said. Paris Prosecutor Francois Molins said the man had been identified, but investigators were still assessing if he had accomplices.
French President Francois Hollande has scheduled an emergency meeting with Prime Minister Bernard Cazeneuve following the shootings.
El Osri's connection with either Cheurfi or the man named in Islamic State's statement remained unclear on Friday.
Police raided the man's home in a Paris suburb after the violence on the bustling, world-famous boulevard. "There were tens, maybe even hundreds of them".
You know, France is three days away from a presidential election, and it is now under - everyone around the world is watching this country, so anyone who wants to get noticed could be doing something like that. "It's America that's not America".
Initial reports had said the incident had left one officer wounded.
It was unclear how Trump concluded that terrorism may have been a factor. "We have to be strong and be vigilant". There have been three major terrorist attacks in the last two years in France.
Pictures of the scene in Paris showed people rushing to clear the area, and a heavy armed police presence.
Police and soldiers closed the streets and subway stations near the site of the shooting.
Brandet told local channel BFM TV that a man came out of his vehicle while it was stopped at a light and started shooting at a police auto.
Two French officials said the gunman was detained towards the end of February after speaking threateningly about the police, but he was then released due to a lack of evidence.
"They were running, running", said 55-year-old Badi Ftaiti, who lives in the area.