England flew into France yesterday (Monday), and into an unprecedented security operation to tackle the twin threats of terrorism and hooliganism at Euro 2016.
Armed police were patrolling the England camp at Chantilly, and France was stunned by the arrest of a French citizen in Ukraine with a "vast arsenal" of explosive and assault rifles. He was described as a right-wing fanatic, who intended to mount up to 15 attacks to coincide with the tournament.
French police say England's first game, against Russia on Saturday in Marseille, where rioting English fans caused widespread damage during the 1998 World Cup, is a particular risk, along with the Poland-Ukraine match on June 21.
Laurent Nunez, the police chief, said: "The terrorist threat is present. We will be very attentive for these high-risk matches."
Ziad Khoury, head of Euro 2016 security, said: "We're ready for everything. There will be incidents but we will be able to react in a coordinated manner."
While the authorities are more concerned about terrorism after the November attacks in Paris, amid fears that radicalisation may be spreading among Marseille's large Arab and Muslim population, English police are working closely with their French counterparts to counter hooliganism.
Although the behaviour of England supporters at international tournaments has improved substantially since 1998, England's return to the Mediterranean city has revived memories of ugly clashes there between visiting supporters and locals from its notoriously tough housing estates.
Police say up to 500,000 Britons will be travelling to the tournament, the closest to home since 1998. They fear they may include some of the 1,110 fans whose banning orders have expired since the 2006 World Cup in Germany.
British police who will travel to Marseille aim to serve as "cultural interpreters" to forestall heavy-handed tactics against fans who may be drunk and rowdy, but unlikely to cause serious trouble. But there is a recognition that there could be problems.
Assistant chief constable Mark Roberts, the lead for UK football policing, said: "Some of the people who've drunk to excess and have behaved in an anti-social manner are not known to the police.
We have up to half a million people travelling; it'll be in summer; -people will no doubt drink; they'll be in large groups. We need to be aware that there may be potential -trouble."
A threat by an English football hooligan nicknamed the 'Pig of Marseille' to team up with right-wing Russian thugs to attack local Muslims in Marseille has received wide coverage in France. James Shayler, 50, acquired the nom de guerre in 1998, when he was jailed in France for leading hooligans who targeted police after England defeated Tunisia.
A particular focus of police operations will be the many small bars in the narrow streets around the Vieux Port, where punches, bottles and stones started flying between groups of local Tunisia fans and England supporters.
One trigger for trouble in the past has been nationalism and Dougie Brimson, a reformed hooligan who has written books about football-related violence, warned of the impact of the referendum over Britain's membership of the European Union. "That is potentially going to manifest itself in quite nasty ways," he said.
French police have identified the England v Russia game as high-risk, and about 1,000 armed police will be deployed at the remodelled Stade Velodrome and the fan zone on the Prado beaches. Extensive use will be made of CCTV cameras.
Helicopters will provide extra surveillance while police boats will patrol the coast. Swimming will be banned from 7pm until 5am, during which time nets will be deployed to prevent bathing.
Inside the stadium, 1,100 security guards - one for every 70 supporters - will watch for potential trouble. Another 650 guards will be deployed inside the fan zone, where crowds of up to 80,000 will watch the match on giant screens.
Amid fears of violence between Russia and England supporters, the fan zone will be divided into two sections with a capacity of 40,000 each and separate screens. Some 400 soldiers and still more police will patrol the city centre.
In all, nine French cities will host Euro 2016 matches. An unprecedented 90,000 police, soldiers and private guards will ensure security.
The Paris fan zone at the foot of the Eiffel Tower has raised -concerns that it could be a magnet for terrorists. However, Bernard Cazeneuve, the French interior minister, -denied a report that the capital's police chief, Michel Cadot, suggested closing the fan zone during matches in Paris.
The minister told the Associated Press that Cadot had informed him "of the conditions under which he could organise it, meaning the extra staff to be perfectly capable of organising security".