A French citizen arrested with a "vast arsenal" of explosives and assault rifles was planning "15 attacks" to coincide with the Euro 2016 football tournament in France, according to Ukraine's state security service.
Confirmation of the arrest of Gregoire Moutaux (25), a suspected far-right nationalist, comes days before Ireland take on Sweden in their Group D opener in Paris on Monday.
Police said Mr Moutaux, who was seized on Ukraine's border with Poland on May 21, had amassed a "vast arsenal" including "more than 5,000 bullets, two anti-tank grenade launchers, 125 kg of TNT, 100 detonators, 20 balaclavas and other things".
News of the plot came as French intelligence said it was "deeply concerned" about a potential attack from homegrown terrorists after the murder of 16 Real Madrid fans this month in Iraq, which agents view as Isil's "postcard" to the West over its murderous intentions for the tournament.
However, Mr Moutaux, who comes from Bar-le-Duc, near Reims, in north-east France, was driven by ultra-nationalist views. His intended targets included Jewish and Muslim places of worship and buildings involved with the soccer tournament, as well as police patrol units, according to police.
French government administration buildings were also a target. He had planned to mine roads and bridges in several regions of France.
"The Frenchman spoke negatively about his government's actions, mass immigration, the spread of Islam and globalisation and also talked about plans to carry out several terrorist attacks," said Vasyl Grytsak, head of the Ukrainian state security service, SBU.
Mr Moutaux, a farm worker, had made contact with armed groups in Ukraine with the aim of buying weapons and explosives. Officers who raided his home found neo-Nazi T-shirts, as well as chemicals used to produce explosives, and five balaclavas. He had no criminal record.
Locals in Nant-le-Petit, which has a population of just 80 people, described Mr Moutaux as a "polite and pleasant lad".
Mr Grystak warned that Mr Moutaux may not have been acting alone because 15 attacks would be impossible for a lone wolf assassin to carry out.
Mr Moutaux's arrest came amid disclosures that 82 members of the 3,500-strong security personnel policing the tournament are on France's intelligence service watch list, which includes Islamist radicals.
Thousands of football fans are due to travel to France for Euro 2016 this week.
The US State Department has warned nationals to remain vigilant of potential attacks in "fan zones" and other public places.
Meanwhile, the threat of disruption from French labour strikes continued last night after hardline unions in SNCF, France's national rail operator, called a seventh straight day of walkouts after talks failed to break the deadlock in the dispute over pay and working hours. (© Daily Telegraph, London)