Paris - France turns its attention on Monday to plugging security holes blamed for failing to prevent the deadliest terrorist attack on the country in half a century, after millions united in historic rallies.In the biggest show of solidarity, in Paris, more than a million people mourned the victims of three days of terror that began with a massacre at the offices of satirical magazine Charlie Hebdo on Wednesday, and ended with 17 people dead.President Francois Hollande will chair a crisis meeting with cabinet ministers on Monday to discuss security measures after the shootings raised questions about how the attackers slipped through the intelligence services' net.All three gunmen - brothers Said, 34, and Cherif Kouachi, 32, and Amedy Coulibaly, 32 - had a history of extremism and were known to French intelligence.Prime Minister Manuel Valls has admitted there were "clear failings" after it emerged that the Kouachi brothers had been on a US terror watch list "for years".Said was known to have travelled to Yemen in 2011, where he received weapons training from al-Qaeda in the Arabian Peninsula, while Cherif was a known jihadist who was convicted in 2008 for involvement in a network sending fighters to Iraq.Coulibaly was a repeat criminal offender who had been convicted for extremist Islamist activity.All three were shot dead by security forces on Friday after a reign of terror that targeted Charlie Hebdo magazine, a kosher supermarket and police.'Not afraid'Hollande has warned his traumatised nation to keep up its guard in the face of possible new assaults.On Sunday he led more than a million people on the march in Paris in tribute to the victims of the attacks as the crowd cried "Not afraid".The interior ministry said nearly four million people took to the streets across the country in the biggest rallies in France's history, with some estimates putting the number in Paris alone at 1.6 million.At the head of the vast and colourful procession in the capital, Hollande linked arms with world leaders including the Israeli prime minister and the Palestinian president, in an historic display of unity.The vast crowd chanted "Charlie, Charlie", in honour of the cartoonists and journalists killed at Charlie Hebdo over its lampooning of the Prophet Mohammed.The crowd brandished banners saying "I'm French and I'm not scared" and, in tribute to the murdered cartoonists, "Make fun, not war" and "Ink should flow, not blood".Emotions ran high in the grieving City of Light, with many people in tears as they came together under the banner of freedom of speech.Isabelle Dahmani, a French Christian married to a Muslim, Mohamed, brought the couple's three young children to show them there was nothing to fear.Firm position Their nine-year-old daughter had burst into tears as she watched TV pictures of the attack on the magazine's offices, Isabelle said, recalling she had asked if "the bad men are coming to our house?"The victims' mourning families played a prominent role in the march, alongside representatives from around 50 countries.In an emotional scene, Charlie Hebdo columnist Patrick Pelloux fell sobbing into the arms of Hollande.With so many world leaders present, security in the still jittery capital was tight, with police snipers stationed on rooftops and plain-clothes officers among the crowd."Today, Paris is the capital of the world," Hollande said. "The entire country will rise up."Hundreds of thousands of people turned out in other French cities including Bordeaux and Lyon, and marches were held in Berlin, Brussels, Istanbul and Madrid as well as in US and Canadian cities.The crowd in Paris was also mourning four Jews killed when Coulibaly stormed a kosher supermarket, after he had earlier gunned down a policewoman.Israeli Prime Minister Benjamin Netanyahu joined Hollande at the main synagogue in Paris after the march to honour the Jewish victims, and praised the "very firm position" taken by French leaders against what he called "the new anti-Semitism and terrorism" in France.'We will win'British Prime Minister David Cameron, who also marched, predicted Europe would face the threat of extremism "for many years to come", but his Italian counterpart Matteo Renzi pledged the continent "will win the challenge against terrorism".Earlier Renzi had tweeted using the hashtag #jesuischarlie (I am Charlie), which has been used more than five million times.Ahead of the rally, interior and security ministers from the European Union and the United States held emergency talks to discuss Islamic extremism.They urged a strengthening of the EU external borders to limit the movement of extremists between Europe and the Middle East, and said there was an "urgent need" to share air passenger information.France's three days of terror started when the Kouachi brothers burst into Charlie Hebdo's offices in central Paris and sprayed bullets into an editorial meeting, killing some of France's best-known cartoonists.They then killed a Muslim policeman as he lay helpless on the ground, and a day later Coulibaly shot dead a policewoman in a Paris suburb.All three gunmen were shot dead Friday after twin hostage dramas at a printing firm and at the kosher supermarket.Investigators have been trying to hunt down Coulibaly's partner, 26-year-old Hayat Boumeddiene, but a security source in Turkey told AFP she arrived there on 2 January, before the attacks, and has probably travelled on to Syria.Coulibaly's mother and sisters condemned his actions, saying "we hope there will not be any confusion between these odious acts and the Muslim religion".