The attack, which left one policeman dead and two others injured, could change the dynamic of the race once more, according to Bruno Jeanbart, head of political studies at pollster OpinionWay.
Voters will cast ballots on Sunday in the first round of what has turned into the most unpredictable French election in memory, with four of the 11 candidates within reach of the two places for the run-off on May 7.
ISIS has reportedly claimed responsibility for a deadly shooting in Paris.
The poll also indicates that if Le Pen and other two frontrunners enter the second round, she will lose by 40 against 60 percent to Melenchon, and 42 against 58 percent to Fillon.
Risky though this may sound for investors, whose confidence is built on the certainty of the single European market and common currency, he is sill not as risky as Le Pen; the National Front candidate is campaigning on a platform of "Frexit" - taking France not only out of the EU but out of the euro too.
Given such demands, markets sense Le Pen holds a much deeper ideological hostility to the European Union than Melenchon, whose principal objection is to the bloc's insistence on austerity policies - a view shared by many more centrist voters.
A man walks past French presidential campaign posters displayed in Sevres, outside Paris, Thursday, April 20, 2017.
"We are in a war that will last: the enemy is powerful, its networks are deep, its accomplices live among us", he said. She said: "This is the end of laxity".
Vimy 100 ceremony at cenotaph draws hundreds
Last week memorial benches to honour the battle of Vimy Ridge were unveiled at the Alliston, Beeton and Tottenham Legion branches. Our victory at Vimy is regarded by many as the moment when Canada stood on its own; as the moment when Canada became a nation.
Like the 2016 U.S. presidential election, the fear of terrorism and the migration crisis have played huge roles in France's political climate. Only the two top-getters will advance to the runoff on May 7.
Addressing 5,000 flag-waving supporters in Marseille, the city where the men were detained, on Wednesday, she said: "I have been denouncing this awful poison of Islamic terrorism since I launched my campaign. and none of my rivals are willing to debate the subject".
Following the shooting, Le Pen said she didn't want the French people to become accustomed to Islamist terrorism.
Experts predict that the worst of all outcomes for the market, a Le Pen-Mélenchon second round, could see the 10-year spread back at its 2011-12 eurozone crisis highs.
Chief investment strategist for Lombard Odier Investment Management Salman Ahmed says investors appear to be penalising Europe based on a perception that its politics are a mess and its economy is mired in a fundamental economic malaise with little hope for improvement.
Even if the positive surprise is not Macron versus Fillon in the second round but Le Pen takes the spot, we believe Macron will go all the way and the reaction function will still be positive.
On Twitter, where automated accounts are allowed, numerous same accounts that promoted Republican Donald Trump in the USA campaign past year have turned their attention to pushing conspiracy theories and far-right viewpoints, according to Limonier and Clinton Watts, a former Federal Bureau of Investigation agent and now a senior fellow at the George Washington University Center for Cyber and Homeland Security. And the elder Le Pen has been expelled from the party.