One of the key questions was whether, and how, the attack that killed one police officer and wounded three other people might impact voting intentions.
Earlier this week, two men were arrested in Marseille whom police said had been planning an attack ahead of the election.
UPDATE: Paris police say that a gunman killed one police officer and wounded another before being killed himself in an attack on the Champs-Elysees shopping district.
The attack came three days before the first round of France's tense presidential election.
Centrist candidate Emmanuel Macron appealed to French voters to keep a cool head.
Paris Prosecutor Francois Molins said the man had been identified, but investigators were still assessing if he had accomplices.More news: Cam Talbot: Cam Talbot gets the hook in loss to Sharks
Hollande, who is not running again for office in an election said: "We shall be of the utmost vigilance, especially in relation to the election".
CNN reported on its website just after 2:30 PM PT that the sole shooter was "known to French security services for radical Islamist activities", and that the immediate "danger is likely over". He said one police officer was killed and two others were seriously injured and hospitalized.
As police blocked off key roadways in the heart of the French capital and told people to avoid the area, the broad avenues leading to the Arc de Triomphe were awash in police vehicles decked with whirling blue lights.
Mr Brandet said the shooting started at shortly after 9pm when a vehicle stopped alongside a police auto at a red light.
French President Francois Hollande says he is convinced the circumstances surrounding the attack on Paris police officers points to a terrorist act. "Traffic has stopped", she says.
Ministry spokesman Pierre-Henry Brandet said on BFM television that a man came out of a auto and opened fire on a police vehicle.More news: Serena Williams's pregnant victory reminds us how incredible women's bodies are
"The perpetrator of the attack in Champs Elysee in central Paris is Abu Yussef the Belgian and he is one of the Islamic State's fighters", it said.
Mr Brandet said a robbery may have been carried out at the same time as the attack but it is not clear if the two are linked.
The two top finishers in Sunday's election will advance to a runoff on May 7. "There were tens, maybe even hundreds of them".
Paris police spokeswoman Johanna Primevert told The Associated Press the gunman targeted police who were guarding the area near the Franklin Roosevelt metro station. Security is high around the vote after France has been attacked in recent years.
A French television station hosting an event with the 11 candidates running for president briefly interrupted its broadcast to report the shootings. The risk for the main candidates was that misjudging the public mood, making an ill-perceived gesture or comment, could damage their chances. She canceled a minor campaign stop, but scheduled another.
French conservative politician Francois Fillon, a candidate in France's presidential election, says campaigning should be suspended following the shooting in Paris.More news: New Executive Order Addresses H-1B Visas
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