Ukraine's security agency has said that a Frenchman detained last month with a large cache of arms was planning mass attacks during the Euro 2016 football tournament, which kicks offs on Friday, the BBC reported.
The man, identified by French media as 25-year-old Gregoire Moutaux,was arrested on the Ukrainian border with Poland.
SBU chief Vasyl Hrytsak said the man had been planning 15 attacks and was driven by ultranationalist views. He had amassed guns, detonators and 125 kg of TNT, Hrytsak said.
Hrytsak listed bridges, motorways, a mosque and a synagogue among the suspect's potential targets. Moutaux was being prosecuted for arms smuggling and terrorism, he said.
It was not clear if the tournament itself was being targeted. News of the man's arrest on May 21 first emerged in a report by French TV network M6.
The suspect was described as a worker at a farming cooperative from the Lorraine area of eastern France. He had no previous criminal record, reports said.
French authorities have been on high security alert ahead of the European championships, amid fears that the tournament could be targeted by Islamist militants.
President Francois Hollande said on Sunday that "the threat exists" but that France should not be daunted.
The SBU said it had been watching the suspect since December last year and that he had picked up five Kalashnikovs, two anti-tank grenade launchers, some 5,000 rounds of ammunition, 100 detonators, as well as a large quantity of explosives.
An SBU video was shown of the dramatic moment of the suspect's arrest along with the weapons that intelligence officials said they had found.
The arrest was said to have taken place at a border crossing close to the Ukrainian town of Yagodyn.
The footage also revealed a second person being wrestled to the ground on the passenger side of the car.
A search was carried out at the suspect's home in the tiny village of Nant-le-Petit and police sources told French media that explosive material and balaclavas were recovered.
An inquiry has been launched by France's organised crime agency, OCLCO, and by regional authorities in Nancy. However, French police sources told AFP news agency that Ukrainian officials had yet to send them any details.