Emmanuel Macron, head of the political movement En Marche!, or Onwards!, and candidate for the 2017 French presidential election, attends a campaign political rally in Paris, France, April 17, 2017.
The gunfire sent scores of tourists fleeing into side streets.
Trump tweeted Friday, "No matter how much I accomplish during the ridiculous standard of the first 100 days, & it has been a lot (including S.C.), media will kill!"
Far-right candidate Marine Le Pen's Front National party has made immigration and the perceived threat of Islam a central plank in her campaign.
Rival François Fillon calls for unity and says France is "at war" against terror.
Under French election rules, Friday was due to be the final day of campaigning before Sunday's first round of voting.
He also wants France to quit North Atlantic Treaty Organisation.
Bernard Cazeneuve called on citizens not to 'give in to extremism'.
Far-right candidate Marine Le Pen said the attack showed the time for naivety "is over". "We have to be strong and we have to be vigilant and I've been saying it for a long time".
Leading presidential candidates have urged greater counterterrorism measures in a campaign already under a state of emergency. We are at war. "This fight for freedom and for the security of the French people must be the priority of the next five-year term".More news: Jupiter and Saturn's moons are ocean worlds
Islamic State claimed responsibility for the attack, according to the group's Amaq news agency.
The attack was the latest targeting security forces at prominent Paris locations.
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.
The attacker emerged from a auto and used an automatic weapon to shoot at officers outside a Marks & Spencer's department store at the center of the Champs-Elysees.
The man "then ran away, managing to shoot and wound two other policemen". Police shot and killed the gunman. One officer was killed and two seriously wounded. He shot two officers in 2001 after being stopped by a police, according to the source. He was convicted in that attack and had a criminal record because of involvement in violent robberies.
French Interior Ministry spokesman Pierre-Henry Brandet told France's Europe 1 radio Friday that it's too early to tell what role the second suspect played, if any, in the attack.
It is not the first terror-related incident to mar the French election cycle, and prior attacks in the country - from the 2015 Paris attacks to last year's Bastille Day attack in Nice - are to the forefront of many voter's minds. Speaking Friday on RTL radio, Macron said: "What our attackers want is death, symbolism, to sow panic (and) to disturb a democratic process, which is the presidential election".
President Donald Trump listens as Italian Prime Minister Paolo Gentiloni speaks during a news conference in the East Room of the White House in Washington, Thursday, April 20, 2017. "What can you say?"
"They really are just focusing now on the micro, which they should be, on the earnings and what the earnings are saying", said Ken Polcari, director of the NYSE floor division at O'Neil Securities in NY.