One of the key questions was if, and how, the attack that killed one police officer and wounded three other people might impact voting intentions.
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".
A man emerged from the vehicle and opened fire on the van with an "automatic weapon", killing one officer instantly, Brandet said.
One police officer and the attacker were killed in the incident. While in detention, he shot and wounded a prison officer after seizing his gun.
Le Parisien newspaper, citing its own sources, reported the solo gunman fired at a police patrol with an automatic weapon.
The attacker is believed to have deliberately targeted the officers, according to reports, and was known to security services.
Reuters had earlier been reporting that several police sources have suggested that it could have been an armed robbery.
Paris police spokeswoman Johanna Primevert told The Associated Press that the attacker targeted officers guarding the area near the Franklin Roosevelt subway station at the center of the shopping avenue popular with tourists.
A televised debate with all 11 presidential candidates was being broadcast when the attack took place.
France is in a state of emergency and at its highest possible level of alert since a string of terror attacks that began in 2015, which have killed over 230 people.
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Paris Prosecutor Francois Molins said the man had been identified, but investigators were still assessing if he had accomplices.
A police helicopter was seen flying low over the area.
Conservative contender Francois Fillon, who has campaigned against "Islamic totalitarianism", said he was cancelling his planned campaign stops on Friday.
Authorities are trying to determine whether "one or more people" might have helped the attacker, Interior Ministry spokesman Pierre-Henry Brandet said.
"I think we must one and all have a spirit of responsibility at this extreme time and not give in to panic and not allow it to be exploited, which some might try to do", he told French radio.
Earlier this week, two men were arrested in Marseille who police said had been planning an attack ahead of the election.
As the first details filtered through, US President Donald Trump sent his condolences and said that "it looks like another terrorist attack".
Emergency vehicles blocked the wide avenue that cuts across central Paris between the Arc de Triomphe and the Tuileries Garden, normally packed with cars and tourists. What can you say? Authorities said a man jumped out of auto and shot at a traffic police vehicle.
The President again weighed in on the attack Friday morning, tweeting, "Another terrorist attack in Paris".
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.