France's 11 presidential candidates were engaged in a live televised debate when the deadly police shooting happened on the Champs Elysees in central Paris.
Analysts had long feared such an attack ahead of the election, following a string of incidents since 2015 that have left over 230 people dead.
The leader of France's nationalist National Front party, Marie Le Pen, says what she calls the "nightmare" is starting again.
Expressing sadness at the loss of life, she has attacked the government and her rivals for not doing more to prevent such attacks.
"I don't want us to get used to Islamic terrorism. I don't want us to say to our young people that they will live daily or long-term with this danger. I want us to put an attack plan in place against this Islamic terrorism, with a series of measures -- borders, but also attacking the root of the evil ... in other words, the ideology itself, which has been festering on our territory for years."
Before the shootings, polls showed voters more concerned about unemployment and their spending power than terrorism or security.
But in the aftermath, analysts have warned that is likely to be different.
In the final days of campaigning, Marine Le Pen hardened her stand against Muslim immigration, linking it to the security fears.
She says she wants to end what she calls "mass immigration" and reassert French cultural identity through a number of measures.
They include a ban on dual nationality for non-Europeans.
Ms Le Pen says she is frustrated to see her worst fears of an attack realised, and she has promised counterterrorism will be a key priority if she is elected president.
"Lenience is over. Naivety is over. We can't leave a weak country to our children. To defend them, we need clarity, we need courage, we need determination, and it's that which you, the French people, must demand and choose."
During the campaign, presidential hopeful Francois Fillon also pledged to eradicate such attacks.
He has called for an alliance with Russia to fight what he calls "Islamic totalitarianism," and he wants to strip French militants returning from the Middle East of their citizenship.
He says fighting terrorism has to be the priority of the next president.
"We are faced with an act that we can't yet totally make sense of, but, sadly, it seems to resemble an act of terror. There seem to have been other acts of violence elsewhere in Paris, and, given the circumstances, I am cancelling my campaign events. And I'd like to say that the fight against terrorism will have to be the absolute priority of the next French president."
Earlier this week, police said two men arrested in Marseille had been planning an attack ahead of the election.
The Paris prosecutor said a machine gun, two handguns and three kilograms of explosives were found at a flat in the southern city, along with militant propaganda.
Candidates in the election said they had been warned about the Marseille attackers.
Another leading candidate, Emmanuel Macron, says the first duty of the president is, and must be, to protect the country.
"We all aspire to become president of the republic, and the first duty of the president is to protect ...This threat is incalculable, and it's going to be a part of our daily lives for years to come."
The former economy minister in Francois Hollande's government has declared himself the ideological opposite of Marie Le Pen.
He says he hopes to convince voters a more progressive government is needed to adapt to the challenges of the future.
Jean-Luc Melenchon, formerly with the Socialist party, joins those three candidates as the frontrunners in Sunday's first round of voting.
The 65 year-old quit the party after 30 years in 2008 and has now started his own, France Unbowed, which has won large support from the French Communist Party.
He has promised to renegotiate France's arrangement with the European Union or leave the bloc altogether.
If no candidate wins more than 50 per cent of the vote on Sunday, the top two advance to a run-off vote on May 7.
