Ms. Le Pen focused her demands on Friday on the roughly 10,000 people that law enforcement officers have flagged as possible Islamist radicals, saying those on the so-called S-files who are foreigners should be deported; those who are dual citizens should be stripped of their French nationality; and 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 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 conservative candidate François Fillon, the leader of the center-right Republicans, said from his campaign headquarters in Paris that France needed to gird 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.”
Mr. Fillon said that, if elected, d he would “take the diplomatic initiative” to reach consensus between Washington and Moscow on destroying the Islamic State, which he vowed to do “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.
The gunman who killed the police officer Thursday evening on the Champs-Élysées in Paris 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 terrorist group 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.
Numerous news agencies 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 for 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, disclosed that the authorities had found a pump-action shotgun, two large kitchen knives, a pair of pruning shears and a Quran had been found in the gunman’s car.
Three people linked to the gunman have been taken into custody for questioning.
The prosecutor’s office also said that the assailant’s car 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 that neighbors Chelles.
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 reports in the French news media 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 proof, 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 the Belgian town of Anvers on Friday morning and did not appear to be related to the Paris case.
Mr. Molins was expected to hold a news conference later on Friday.