A Frenchman with an arsenal of weapons and a grudge against his government's immigration policies was detained as he tried to leave Ukraine and launch an assault on the Euro 2016 football tournament, Ukrainian officials have said.
They described it as a thwarted plot for a "series of 15 terrorist attacks".
The announcement came as France promised unprecedented security for the tournament's crowds and stadiums, which includes the French national stadium that was attacked by suicide bombers on November 13.
The event begins on Friday among Europe's top 24 national teams and lasts a month.
Ukrainian agents had been following the man since December, and allowed him to buy five machine guns, two rocket-propelled grenade launchers, 125kg of TNT, 100 detonators and other arms. He also bought 20 balaclavas before he was arrested at the Yahodyn border crossing between Ukraine and Poland last month, the Ukrainian Security Service said.
The intelligence agency, known as the SBU, "has managed to foil a series of 15 terrorist attacks which were planned to target France before and during" Euro 2016, said its chief, Vasyl Hrytsak.
Ukraine said the suspect wanted to protest against his government's immigration policies and the spread of Islam. It said he planned attacks on bridges, railways and other infrastructure for the tournament.
One expert said the sources of potential attackers against Euro 2016 had expanded with the news from Ukraine.
"Just because you have stopped one group does not mean there's not another one elsewhere," David Benichou, a French anti-terrorist judge, told RTL radio. "The security forces are largely focused on a menace that is known, is evident, that doesn't hide and that has threatened us specifically. But we cannot neglect the other potential sources of terrorism, like the extreme right."
The Paris prosecutor's office, which handles national terrorism cases, said no investigation had been opened yet. France's foreign ministry confirmed the arrest in Ukraine, but offered no further details.
Extremist attacks are a major concern for French authorities as they prepare to host the tournament at stadiums in the Paris area and eight other French cities through to July 10.
Islamic State extremists have threatened France during the tournament but authorities have not confirmed specific dangers.
France is deploying a 90,000-strong security force, and President Francois Hollande said the threat of attacks will not stop the tournament from being successful.
The Paris police chief, Michel Cadot, has asked for more security personnel in the French capital, however.
Mr Cadot declined to comment on the information from Ukraine, saying only "there is no specific threat against any (Euro 2016) site".
French regional newspaper L'Est Republicain identified the man as Gregoire Moutaux and said investigators raided his home in Nant-le-Petit near the eastern city of Nancy in May. French television network M6 reported that investigators found T-shirts promoting an extreme-right group.
Ukrainian authorities released photos of a fair-haired man, his face blurred, holding various weapons, as well as a video of the arrest, which showed special forces officers dragging him from a white minivan and putting him face down on the ground.
The Frenchman arrived in eastern Ukraine last year and was "trying to establish ties with Ukrainian troops under the guise of volunteering", the Ukrainian agency said.
Ukrainian troops and Russia-backed separatists have fought in eastern Ukraine since April 2014, killing nearly 9,400 people. It was not immediately clear which side of the conflict the Frenchman had been on.
Mr Cadot said security measures tested since the November 13 attacks in Paris that killed 130 victims have been adapted to fit Euro 2016, creating more access points, security checks, bag and ticket controls and some pat-downs.
"We needed to improve," Mr Cadot told a press conference. "We want to avoid the threat of having someone carrying (in) explosives."
He insisted on more fluidity at the stadium's entrance after fans complained about severe overcrowding at security checkpoints at the Stade de France in Paris last month. Despite a two-metre security wall surrounding the stadium and triple security checks, supporters smuggled flares into the venue and ignited fires in the stands. Some supporters also tried to invade the field.
Richard Walton, head of the counter-terrorism command at the Metropolitan Police during the 2012 London Olympics, said France's challenge was enormous.
"Successful counterterrorism is ultimately only measured by the absence of a terrorist incident," Mr Walton wrote. "An attack anywhere in France during Euro 2016 is an attack on the event itself."