PARIS — President 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 American presidential race last year. It was not clear, however, that Mr. Trump’s statement would help her among undecided voters.
In a statement on Friday, Ms. Le Pen blamed “radical Islam” — “a monstrous, totalitarian ideology that has declared war on our nation, on reason, on civilization” — for the attack on Thursday night.
Ms. Le Pen focused her demands on the roughly 10,000 people that law enforcement officers have flagged as possible Islamist radicals, saying that those on the so-called S-files who are foreigners should be deported; that those who are dual citizens should be stripped of their French nationality; and that those who are French should be prosecuted.
Legal experts have noted, however, that the threshold for being designated for the S-files is very low compared with the evidence needed to secure a criminal conviction.
The conservative candidate François Fillon said that France needed to prepare for a long struggle.
“We are in a war that will be long,” he said in an address from his campaign headquarters. “The opponent is powerful; its networks are numerous; its accomplices live among us and beside us.”
Mr. Fillon said that if elected, he would “take the diplomatic initiative” to broker consensus between Washington and Moscow on destroying the Islamic State “with an iron hand.” He added that “France’s Muslims overwhelmingly want to live their faith in peace,” and appealed for their help in combating fundamentalism.
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 mobilized every day along France’s frontiers since the attacks in and around Paris on Nov. 13, 2015. He also noted that 117 people had been expelled from France over 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.”
He said it was hard to believe Mr. Fillon’s promise of 10,000 new police jobs, saying that Mr. Fillon, when he was prime minister from 2007 to 2012, had overseen spending reductions that resulted in the loss of 54,000 military jobs and 13,000 internal security jobs. During that period, France, like many countries, was tightening its belt in response to the financial crisis.
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.”
The gunman who killed the officer on Thursday evening, on the Champs-Élysées, 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, who was born in 1967 and lived in Chelles, an eastern suburb of Paris. In an interview on Friday, a law enforcement official who spoke on the condition of anonymity because the investigation was continuing confirmed that Mr. Cheurfi was the gunman, and that he had been convicted around 2003 of attempted murder, after he shot two police officers.
Mr. 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.
On Friday, the office of the Paris prosecutor, François Molins, said that the authorities searching the gunman’s car had found a pump-action shotgun, two large kitchen knives, a pair of pruning shears and a Quran.
Three people linked to the suspect have been taken into custody for questioning.
The prosecutor’s office also said that the assailant’s car had contained a piece of paper with the addresses of the French domestic intelligence agency and of a police station in Lagny-sur-Marne, a town about 13 miles east of Paris.
Pierre-Henry Brandet, a spokesman for the Interior Ministry, told Europe 1 Radio on Friday that the police officers who killed the gunman had averted a “blood bath, a carnage on the Champs-Élysées.”
“This was an individual who was known by the judiciary, who was known to police services, who was a dangerous individual,” he said.
Asked about news reports that the assailant had been briefly arrested in February after he expressed his intention to kill police officers, but that he had been released for lack of evidence, Mr. Brandet declined to comment.
On Thursday, Belgian authorities issued a warrant for what they believed was a dangerous man who intended to travel to France. Some news reports suggested Thursday evening that the man might be connected to the shooting on the Champs-Élysées, but Mr. Brandet said the man turned himself in at a police station in Antwerp, Belgium, on Friday morning and did not appear to be related to the Paris case.
Belgium’s interior minister, Jan Jambon, said on Friday that “the guy who did the act yesterday was not a Belgian, he was French.” The Islamic State had referred to the attacker using a pseudonym, Abu Yusuf Al-Beljiki, that described him as “The Belgian.”