Paris: President Donald Trump inserted himself into the tumult of French politics on Friday, declaring that the fatal shooting of a police officer in central Paris would have "a big effect" when voters go to the polls on Sunday to choose among 11 presidential candidates.

Mr Trump did not mention any candidates by name. But his statement on Twitter - "The people of France will not take much more of this. Will have a big effect on presidential election!" - came at the tail end of a tight, fragmented race, with at least four contenders running neck and neck.

One of them, Marine Le Pen, the far-right candidate, has issued grim warnings that a declining France is losing its identity, echoing Mr Trump's themes during the US presidential race last year. It was not clear, however, that Mr Trump's statement would help her among undecided voters.

In a statement Friday, Ms Le Pen blamed "radical Islam" - which she called "a monstrous, totalitarian ideology that has declared war on our nation, on reason, on civilization" - for the attack on Thursday night.

The conservative candidate Francois Fillon said from his campaign headquarters in Paris that France needed to prepare for a long struggle.

"We are in a war that will be long," he said. "The opponent is powerful; its networks are numerous; its accomplices live among us and beside us."

The centrist independent candidate Emmanuel Macron urged France not to succumb to the fear that extremists seek to spread.

"They want France to be afraid; they want to disrupt the democratic process; they want the French to yield to unreasonableness and division," he said. "Our challenge is to protect the French, not to give up who we are, to stay unified and build a future."

Prime Minister Bernard Cazeneuve responded to Ms Le Pen and Mr Fillon with a point-by-point rebuttal.

"She has pretended to ignore that it was this government that restored border controls," he said of Ms Le Pen, noting that more than 2,300 officers had been mobilised every day along France's frontiers since the attacks in and around Paris on November 13, 2015. He also noted that 117 people had been expelled from France for terrorist activities, and that Ms Le Pen's party had voted against laws that strengthened the government's intelligence-gathering powers.

"For all of our citizens, for our entire country, this attack is a tragedy," he said. "Ms Le Pen seeks to make it an opportunity."

Len Pen gains in polls but Macron leads

Ms Le Pen has gained some support in the first opinion poll to be held entirely after the Paris attack, but Mr Macron was still seen beating her on Sunday and defeating her in a runoff two weeks later.

In an Odoxa poll for Le Point published on Friday, Mr Macron was seen winning the first round with 24.5 per cent of the votes, down 0.5 percentage points compared to the previous poll. Ms Le Pen took the second place in the poll with 23 per cent, up 1 point.

Mr Fillon and Communist-backed Jean-Luc Melenchon were both down 0.5 points to 19 per cent, hence failing to qualify for the May 7 runoff.

Islamic State claims attack

The gunman who killed the officer on Thursday evening on the Champs-Elysees had been detained in February for threatening the police, but he was released because there was not enough evidence to charge him, according to French news agencies and a law enforcement official.

The Islamic State claimed responsibility within hours of the attack, which also wounded two police officers and a bystander and briefly shut down the city's most famous boulevard. The gunman was shot dead by the police as he tried to flee.

Several news outlets identified the gunman as Karim Cheurfi. In an interview on Friday, a law enforcement official who spoke on the condition of anonymity because the investigation was continuing confirmed that Cheurfi was the gunman, and that he had been convicted around 2003 of attempted murder, after he shot two police officers.

Cheurfi was sentenced to 20 years in prison, but the sentence was reduced to 15 years and he served less than that, the official said.

Three people linked to the suspect have been taken into custody for questioning.

The New York Times, AP