PARIS — President François Hollande of France called on Monday to amend the constitution to fight potential terrorists at home and for an aggressive effort to "eradicate" the Islamic State abroad.
His call to arms — "France is at war," he said at the opening of his remarks to a joint session of the parliament — came as security forces in France and Belgium zeroed in on a suspect they said was the architect of the assault that killed 129 people Friday night in Paris. The suspect, a 27-year-old Belgian, has fought for the Islamic State in Syria and has been linked to other terrorist attacks.
Hollande spoke after the French police raided homes and other sites across the country in an effort to head off possible further attacks and the authorities in Belgium hunted for a suspected assailant in Friday's attacks.
Hollande called for quick action by the parliament on new legislation that would give the government more flexibility to conduct police raids without a warrant and place people under house arrest. He said he would seek court approval for broader surveillance powers. And he called for constitutional amendments that would give more weight to security measures relative to civil liberties.
The changes he is seeking would, among other things, extend the current state of emergency for three months and let the government strip the citizenship of French natives who are convicted of terrorism and hold a second passport.
"Our democracy has prevailed over much more formidable opponents than these cowardly assassins," Hollande said a day after France conducted airstrikes against the Syrian city of Raqqa, the self-proclaimed capital of the Islamic State. It was the country's most intense military strike yet against the radical group, which had claimed responsibility for the attacks in Paris.
The French leader said he would meet soon with President Barack Obama and President Vladimir Putin of Russia in an effort to settle on a united campaign to wipe out the Islamic State, also known as ISIS or ISIL.
"Terrorism will not destroy the republic, because it is the republic that will destroy it," he said.
Three days after the attacks on a soccer stadium, a concert hall and numerous bars and cafes, French and Belgian security services were focused on the radical jihadi they believe was the leader of the plot, Abdelhamid Abaaoud. He is among the most prominent Islamic State fighters to have come out of Belgium.
A French official briefed on the investigation, who spoke on the condition of anonymity because he was not authorized to discuss operational details, said Abaaoud had mentioned plans to attack "a concert hall" to a French citizen who returned from Syria.
Abaaoud, this official said, had also been in contact with Ismael Omar Mostefai, one of the Paris attackers. Abaaoud also knew another attacker, Ibrahim Abdeslam; they were tried together in 2010 in Belgium for a minor offense.
Hollande said the attacks had been "planned in Syria, organized in Belgium, perpetrated on our soil with French complicity."
The French authorities said Monday that they had conducted 168 raids across the country in an effort to root out possible terrorist threats. The raids extended from the Paris region to the major cities of Lille, Lyon, Marseille and Toulouse, they said. They also said they had arrested 23 people and detained 104 others under house arrest.
But a Frenchman believed to be involved in the Paris attacks, Salah Abdeslam, 26, a brother of Ibrahim Abdeslam, remained at large, eluding a series of raids conducted by the authorities in Molenbeek, the working-class Brussels neighborhood where the brothers lived.
A third brother, Mohamed, and four other men who had been detained in Belgium were released on Monday. At a news conference in Brussels, Mohamed said he did not know Salah Abdeslam's whereabouts and added, "My parents are under shock and have not yet grasped what has happened."
The alleged architect of the plot, Abaaoud, who traveled to Syria last year and even persuaded his 13-year-old brother to join him there, is from the same neighborhood, Molenbeek, as the Abdeslam brothers.
Abaaoud was already a suspect, according to officials and local news reports, in a failed terrorist plot in Belgium in January and an attempt in August to gun down passengers on a high-speed train to Paris from Brussels. The intelligence official said the authorities feared he might be in Europe.
At noon, France observed a moment of silence in honor of the victims of the attack, which wounded about 350 people, in addition to the 129 killed. The Métro and cars stopped and crowds gathered at a makeshift memorial at the Place de la République and at the Eiffel Tower. Hollande stood with students at the Sorbonne. Many recited the national anthem, "La Marseillaise," after the moment passed. In other cities — Delhi, Doha and Dublin — crowds gathered at French embassies to pay their respect.
As France observed its second of three days of national mourning, the authorities in both France and Belgium raced to track down suspects and chase leads.
At one house in the Rhône department in the southeast, around Lyon, the police found a Kalashnikov assault rifle, three automatic pistols, ammunition and bulletproof vests. Officers then obtained a warrant to search the home of the parents of a man who lived in the house, where they found several automatic pistols, ammunition, police armbands, military clothing and a rocket launcher.
Prime Minister Manuel Valls and Interior Minister Bernard Cazeneuve promised to keep up the search.
"We are using all the possibilities given to us by the state of emergency, that is to say administrative raids, 24 hours a day," Valls said, vowing to keep intense pressure on "radical Islamism, Salafist groups, all those who preach hatred of the Republic."
The authorities confirmed on Monday that one of the attackers had entered Europe through Greece on a Syrian passport last month, posing as a migrant.
The man was identified on his passport — found at the soccer stadium north of Paris where he blew himself up on Friday night — as Ahmad al-Mohammad, 25, a native of Idlib, Syria. The holder of the passport passed through the Greek island of Leros on Oct. 3 and the Serbian border town of Presovo on Oct. 7, according to Greek and Serbian officials. It remained unclear if the passport was authentic.
All told, at least four French citizens were among the seven attackers. Ibrahim Abdeslam; Mostefai, who met with the suspected planner of the attacks; and two men identified on Monday as Samy Amimour, 28, a Paris native who lived in the suburb of Drancy, and Bilal Hadfi, 20, who lived in Brussels.
Amimour was known to the French authorities, having been charged in October 2012 with terrorist conspiracy, according to the authorities. He was placed under judicial supervision but violated the terms of that supervision in the fall of 2013, prompting the authorities to put out an international arrest warrant.
In December, the French newspaper Le Monde had interviewed Amimour's father — it did not identify him by name at the time — who had gone to Syria to try to bring back his son. Three members of the Amimour family were detained on Monday.
The Turkish government confirmed on Monday that Mostefai, 29, had entered Turkey in 2013 but said "there is no record of him leaving the country."
A senior Turkish official, speaking on the condition of anonymity, said that the government flagged Mostefai twice — in December and again in June — but that "we have, however, not heard back from France on the matter."
He continued, "It was only after the Paris attacks that the Turkish authorities received an information request about Ismael Omar Mostefai from France."
The official added that "this is not a time to play the blame game" but added that governments needed to do better at sharing intelligence to prevent terrorism.
The United States has provided logistical support for the French airstrikes in Syria, but Obama on Monday again ruled out a ground intervention. "Let's assume that we were to send 50,000 troops into Syria," he said at a gathering of leaders of the Group of 20 industrial and emerging-market economies in Antalya, Turkey. "What happens when there's a terrorist attack generated from Yemen? Do we then send troops into there? Or Libya, perhaps?"
Elsewhere in Europe, the authorities tightened security. Britain on Monday announced that it would pay for an additional 1,900 intelligence officers, and review aviation security, as part of its response to the attacks.
The home secretary, Theresa May, said there would be tighter surveillance of those arriving in Britain and that border guards were making targeted checks of passengers and vehicles leaving for France.
Prime Minister David Cameron, who like Obama attended the Group of 20 meeting, said he would consider speeding up the legislative timetable for a proposed law to govern electronic surveillance by the intelligence agencies, though he added that it was important to bring Parliament and public support with him.
In Washington, John O. Brennan, the director of the CIA, said Monday that both the Paris attacks and the crash of a Russian jet over the Sinai Peninsula bore the "hallmarks" of the Islamic State.
Speaking at the Center for Strategic and International Studies, Brennan called the group an "association of murderous sociopaths" that is "not going to content itself with violence inside the Syrian and Iraqi borders."
Wading into the debate over surveillance, privacy and encryption, Brennan said he hoped the Paris attacks would be a "wake-up call," adding "hand-wringing" had weakened the ability of Western intelligence services to prevent attacks.