A suspected Isis supporter opened fire with a Kalashnikov on the famous Champs Elysees, killing one officer and injuring two others before being shot dead.
Coming three days before the first round of France’s presidential election, there has been speculation that increased security fears could drive voters to either far-right leader Ms Le Pen or Mr Fillon.
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Donald Trump was among those predicting the atrocity would have a “big effect” on the election, while the French Prime Minister attacked candidates for seeking to turn it into a political opportunity.
Bernard Cazeneuve launched an unprecedented intervention, saying it was “necessary to re-establish the truth”.
He accused Ms Le Pen of seeking to divide France, while picking apart Mr Fillon’s policies.
The two candidates and frontrunner Emmanuel Macron cancelled planned events on Friday, which would have been the last day of campaigning.
But Ms Le Pen chose to launch a high-profile speech in an apparent attempt to gain political capital from the bloodshed, hitting out at France’s “so-called leaders” for failing to protect citizens.
“Islamism is a monstrous totalitarian ideology that has declared war on our nation, on reason, on civilisation,” the Front National leader said.
“The names of these new victims are added to the long list of martyrs of terrorism.”
She called for France to reinstate border checks and to arrest all suspects on France’s “fiché S” terror watch list, while deporting foreign suspects and removing French citizenship from dual nationals.
Following reports that the suspected attacker had been arrested as part of a counter-terror investigation in February but released the following day, Ms Le Pen called him a “symbol of [state] failure”.
She made no mention of the fact the chief suspect is a Frenchman, but denounced the “laxity of the judiciary” and announced proposals to stop Islamism spreading in France, including banning Salafist organisations, expelling hate preachers and stopping the foreign financing of mosques.
Ms Le Pen, who has moved her party into the mainstream after her anti-Semitic father’s leadership, claimed a war was being waged aiming to “subject France to a totalitarian ideology”.
“This war is being waged without pity and without respite… the response must be total across the entire country,” she added.
“I call for the awakening of our people’s ancient soul, capable of opposing a bloodthirsty barbarism.”
She was speaking after facing off against 10 other presidential candidates in a final television debate on Thursday night, with the attack taking place as they were on air.
Gunman attack on the Champs-Elysees
For several months, polls have forecast Ms Le Pen and Mr Macron to progress to the second round of the election, with the centrist former finance minister expected to easily beat the radical candidate.
But debates have seen the left-wing candidate Jean-Luc Melenchon enjoy a surge in support, amid a recovery by the former Prime Minister Mr Fillon, whose chances were damaged by a scandal involving payments to his wife.
Voting intention was on a knife-edge on Thursday morning, with a chart for The Independent by Statista showing 24 per cent supported Mr Macron, 22.5 per cent for Ms Le Pen and 19.5 per cent for Mr Fillon.
Analysts have predicted the latest terror attack – coming after the murder of almost 240 people in a series of Isis-related atrocities in France – could drive voters to the right wing.
While Ms Le Pen could profit, pushing her radical policies on immigration, security and Islamic fundamentalism, anxious voters could also turn to Mr Fillon as a more experienced pair of hands.
He has also put security centre stage in his campaign, vowing to increase police officers by 10,000 and prison places by 16,000, while stripping French jihadis who fight abroad of their nationality and banning them from returning to France.
François Fillon was considered a frontrunner before the “fake jobs” scandal (REUTERS)
Like Ms Le Pen, he has also proposed an overhaul of the Shengen agreement that allows border-free movement through Europe and called for a new European border police force and an annual immigration quota.
Mr Fillon echoed his rival in claiming that “Islamist totalitarianism” should be the priority of France’s next president.
“Radical Islam is challenging our values and our strength of character,” he told reporters.
“We are at war, there is no alternative, it’s us or them.”
Striking a more conciliatory tone, frontrunner Mr Macron appealed to French voters to keep a cool head, telling RTL radio: “What our attackers want is death, symbolism, to sow panic and to disturb a democratic process, which is the presidential election.”
He vowed that, if elected, he would create a task force to coordinate French intelligence efforts against Isis within weeks of taking power.
Thursday’s attack came days after police foiled an “imminent” bombing plot believed to be linked to the election.
Isis flags were found at the suspects’ homes and the group also claimed responsibility for the Champs Elysees shooting, identifying the attacker as a “fighter of the Islamic State” using the war name Abu Yusuf al-Baljiki.
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It indicates a link to Belgium but the country’s interior minister said the suspect was a French national, amid searches at the home of a Frenchman previously convicted for shooting two police officers.
Karim Cheurfi, 39, was reportedly the subject of another counter-terror investigation in February but was released because of a lack of evidence.
A Belgian man also handed himself into police in Antwerp after being identified as a suspected accomplice, but was not found to be linked to the attack.
Investigators found a pump-action shotgun and knives in the car of the gunman, who pulled up behind a police van and opened fire with a Kalashnikov.
One officer was killed and two more injured, as well as a German woman whose foot was wounded.
It was one of almost a dozen attacks and plots targeting the French security services since 2012, including the murder of a police officer and his wife by an Isis supporter north of Paris, a machete attack outside Le Louvre and an attempted murder at Orly airport.
The Prime Minister said the government has reviewed its already extensive election security measures and says it is “fully mobilised” for Sunday’s vote, deploying 50,000 police and 7,000 soldiers.
“Nothing must hamper this democratic moment, essential for our country,” Mr Cazeneuve said, appealing for national unity and for people “not to succumb to fear.”