A Frenchman detained last month with a large cache of arms was planning mass attacks during the Euro 2016 football tournament, which starts on Friday, Ukrainian officials say.
The man, identified by French media as Gregoire Moutaux, 25, was arrested on the Ukrainian border with Poland.
Intelligence chief Vasyl Hrytsak said the man had planned 15 attacks and was driven by ultra-nationalist views.
He had amassed guns, detonators and 125kg of TNT, Mr Hrytsak said.
Mr Hrytsak listed bridges, motorways, a mosque and a synagogue among the suspect’s potential targets. He was being prosecuted for arms smuggling and terrorism, he said.
It was not clear if the tournament itself was being targeted and Paris police prefect Michel Cadot told reporters there was “no specific threat against any [Euro 2016] site”.
News of the man’s arrest on 21 May first emerged in a report by French TV network M6. The suspect was described as a worker at a farming co-operative 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.
Analysis – by BBC Paris correspondent Hugh Schofield
The French police and judicial authorities are saying very little about Gregoire Moutaux. They clearly want to know more from the Ukrainians before they pronounce on any possible terrorist link.
What’s significant is that so far the French investigation is not in the hands of anti-terrorist specialists, but of the unit that looks into organised crime.
This would suggest that the initial suspicion in Paris is that Mr Moutaux was engaged in arms trafficking.
His home in north-east France has been searched by police, who – according to media reports – found a T-shirt bearing far-right insignia. The Ukrainians say he was motivated by hatred of immigration and globalisation.
Ukraine’s SBU security service 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 and 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 Yahodyn.
The footage also revealed a second person being wrestled to the ground on the passenger side of the car.
The SBU chief said the French suspect had been in touch “with military units fighting in Donbass”, a reference to the eastern areas of Luhansk and Donetsk, where pro-Russian rebels have seized large areas of Ukrainian territory.