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 Grégoire Moutaux, 25, a suspected far-Right nationalist, came on the day the England team arrived at their French base ahead of their first game in Marseille on Sunday.
Police said Moutaux, who was seized on Ukraine's border with Poland on May 22, had amassed a "vast arsenal" including "more than 5000 bullets, two anti-tank grenade launchers, 1
News of the plot came as French intelligence said it was "deeply concerned" about a potential attack from home-grown terrorists following the slaying of 16 Real Madrid fans this months in Iraq, which agents view as Isis' "postcard" to the West over its murderous intentions during the tournament.
However, Moutaux, who originally comes from Bar-le-Duc, near Reims, in northeast France, was driven by ultra-nationalist views.
His intended targets included Jewish and Muslim places of worship and buildings involved with the football tournament, as well as police patrol units, according to police.
French government administration buildings, including those dealing with tax collection, were also a target.25 kg of TNT, 100 detonators, 20 balaclavas and other things".
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.
Moutaux, a farm worker, had made contact with armed groups in Ukraine with the aim of buying weapons and explosives.
"The SBU was able to prevent a series of 15 acts of terror (planned) for the eve and during the Euro soccer championship," said Mr Grystak, who added that the SBU had sold Moutaux deactivated weapons after they found out that he was looking to purchase the arms.
Soon after Moutaux's arrest in Ukraine, a raid was carried out at his home in France. There officers 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 Moutaux as a "polite and pleasant lad".
Thousands of football fans are due to travel to France for Euro 2016 this week, amid warnings that they face an unprecedented security threat.
French intelligence has been on high alert since Isis operatives killed 130 people in November with suicide bombers exploding their devices around the Stade de France during a football friendly between France and Germany.
Suicide bombers then killed 32 people at Belgium's main international airport and on the Brussels metro in March.
Plans to launch fresh attacks in France during Euro 2016 emerged on a computer belonging to Salah Abdeslam, the only surviving member of the Isis Paris death squad.
The arrest came amid revelations that 82 members of the 3500-strong security personnel employed to police sensitive Euro sites are on France's intelligence service watch list, which includes potentially violent Islamist radicals.
The figure was in a note from the DGSI, France's domestic spy agency, leaked to Le Point magazine.
Speaking to AFP, one French intelligence agent warned: "Quite frankly, I'm worried."
"What we're most concerned about are people already in place in Europe, which is apparently the case," he said.
"Guys stationed in Germany, for example, who we didn't see get through, who the Germans didn't spot and who have been lying in wait ever since. We've re-established certain borders, but let's be honest, borders are uncontrollable."
The intelligence source said that the Isis attack on May 14 on local Real Madrid supporters gathered to watch old matches and 16 of whom were gunned down was a "post card sent to the Euro".
"It's a direct message. It was perhaps meant to frighten us, in which case it has succeeded in doing so," he said.
One terror attack would see three quarters of the Euro teams go home, and a pledge of fresh ones would kill the tournament altogether, he warned.
Security officials are concerned about "soft targets", in particular fan zones - cordoned off areas in ten cities around France where supporters can watch the games on giant screens.