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.
"The Frenchman spoke negatively of the activities of his government on mass migration of foreigners to France, the spread of Islam and globalisation. He also said he wished to stage a number of terrorist attacks in protest," Mr Hrytsak said.
Euro 2016 security
- 90,000 police and other security officials to patrol fan zones and stadiums
- Paris to have security force of at least 13,000 to patrol two zones and two stadiums
- Seven million people expected to visit 10 French cities from Lille in the north to Marseille in the south
- State of emergency in place after last November's jihadist attacks in Paris in which 130 people died
- Officials deny a report that Paris prefect Michel Cadot asked for the Eiffel Tower fan zone to be shut
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. There was some scepticism that the suspect could have been anything more than an arms trafficker.
Residents in Nant-le-Petit were stunned by the arrest of Gregoire Moutaux, who lives there with his grandfather and works as a cattle inseminator.
"He's a rather nice guy, who I think is actually rather well-educated. I've even invited him over for a glass of wine. I just can't understand this," neighbour Jean-Jacques Renck told France 3 TV.
Anti-terrorism specialist Alain Bauer said Ukraine had become a big source of arms because of the continuing conflict in the east of the country.
The man detained did not fit the usual profile of an attacker, he said, pointing to the arrest last year of far-right activist Claude Hermant, accused of providing weapons to a jihadist who murdered four people at a Jewish supermarket in Paris in January 2015.