Police raided homes of suspected Islamist militants across France overnight arresting 23 people, and investigators identified a Belgian national living in Syria as the possible mastermind behind Friday's attacks in Paris.
Much of France came to a standstill at midday for a minute's silence to remember the 129 killed in the co-ordinated suicide bombings and shootings. Metro trains stopped, pedestrians paused on pavements and office workers stood at their desks.
Prosecutors have identified five of the seven dead assailants -- four Frenchmen and a foreigner fingerprinted in Greece last month. His role in the carnage has fuelled speculation that Islamic State took advantage of a recent wave of refugees fleeing Syria to slip militants into Europe.
Police believe one attacker is on the run, and are working on the assumption that at least four people helped organise the mayhem, the worst atrocity in France since World War Two, which appears to have been organised in neighbouring Belgium.
Belgian police arrested at least one person after a four-hour siege on Monday at a house in the Brussels district of Molenbeek, home to many Muslim immigrants, but failed to find a man believed to have played a key role in the assault.
[Check photos: World reacts to Paris attacks]
"We know that more attacks are being prepared, not just against France but also against other European countries," French Prime Minister Manuel Valls told RTL radio. "We are going to live with this terrorist threat for a long time."
Islamic State, which claimed responsibility for the attacks in retaliation for French airstrikes in Iraq and Syria, warned in a video on Monday that any country hitting it would suffer the same fate, promising specifically to target Washington.
French warplanes bombed Islamic State training camps and a suspected arms depot in its Syrian stronghold Raqqa late on Sunday -- its biggest such strike since it started assaults as part of a U.S.-led mission launched in 2014.
Interior Minister Bernard Cazeneuve told reporters police had arrested nearly two dozen people and seized arms, including a rocket launcher and automatic weapons, in 168 raids overnight. Another 104 people were put under house arrest, he said.
"Let this be clear to everyone, this is just the beginning, these actions are going to continue," Cazeneuve said.