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Leading candidates on the right in France's presidential vote have been accused of using the killing of a policeman in Paris for political ends.
The far right's Marine Le Pen pledged to expel radical Islamists while the centre-right's Francois Fillon talked of fighting "Islamist totalitarianism".
Prime Minister Bernard Cazeneuve accused Mr Fillon and Ms Le Pen of cynically exploiting the attack.
Friday is the last day of campaigning before Sunday's election.
Ms Le Pen, Mr Fillon and independent centrist Emmanuel Macron all cancelled their final campaign events as a mark of respect for the policeman killed on Thursday.
However, radical leftist Jean-Luc Mélenchon, who also condemned the attack, pressed on with his campaigning, saying "the violent must be shown that they will not have the last word against republicans".
In an attack claimed by so-called Islamic State, a gunman shot dead a policeman on the French capital's famous Champs Elysees Avenue, wounding two other officers before being killed by security forces.
The gunman struck as the 11 candidates took part in their last TV appearance together before the election, a tightly fought contest to be followed by a run-off on 7 May.
Opinion polls suggest Mr Macron has a narrow lead over Ms Le Pen, with Mr Fillon and Mr Mélenchon close behind.
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Ms Le Pen, leader of the National Front (FN), called for foreigners on security watch lists to be expelled and for national border controls to be reinstated immediately.
"We must fight barbarism - none of the French governments for the past 10 years has done enough," Ms Le Pen said.
Speaking of "incredible lapses in the justice system", she said that both the current government and its predecessor had done everything to make France lose the war on terrorism.Image copyright Getty Images Image caption The policeman was shot near the Marks and Spencer store on the Champs Elysees
Mr Fillon, the former prime minister who is standing for the Republican party, said France needed to show its enemies it was united and without fear.
He implicitly criticised the Socialist government when he said that some had not "taken the full measure of the evil", the Eyes On Events's Hugh Schofield reports from Paris.
He himself promised an "iron-fisted" approach.
Mr Cazeneuve criticised Mr Fillon's record on security when he was prime minister and accused Ms Le Pen of seeking to "exploit fear without any shame".
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Mr Macron said the first task of a president was to protect French citizens and he would be "implacable" in combating the Islamist threat.
Tweeting after Thursday's TV appearance, Mr Mélenchon said: "Terrorist attacks will never go unpunished, accomplices never forgotten."Image copyright AFP Image caption After Sunday just two candidates will remain in the race
On Friday, he was joined by fellow radical leftist Pablo Iglesias, leader of Spain's Podemos movement.
In Washington, US President Donald Trump, whose election win stunned much of the world last year, predicted in a
that the attack would "have a big effect on [the] presidential election".
Another terrorist attack in Paris. The people of France will not take much more of this. Will have a big effect on presidential election!— Donald J. Trump (@realDonaldTrump) April 21, 2017
No-one knows how the attack will play on the election, our correspondent says, but the killing of a policeman on the Champs Elysees is a shocking event that will certainly be on people's minds as they contemplate their choice this weekend.
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You can follow the first round of the French election on the Eyes On Events News website. Click here for all our latest coverage. On the day of the election, we will be running a live page bringing together the latest news, video and analysis.
On TV, you can watch a Eyes On Events World News Election Special, from 18:30 BST (17:30 GMT / 19:30 local time in France) on Sunday, which will be broadcast on Eyes On Events News in the UK and on Eyes On Events World News internationally, with Christian Fraser presenting from Paris.
For radio, Eyes On Events World Service will broadcast a special extended edition of Newshour from Paris at 18:00 GMT on Sunday.