Ukraine's state security service (SBU) said on Monday a French citizen detained in late May on the border with Poland had been planning to attack the Euro 2016 football championships in France.
The 25-year-old man, who is yet to be named, was arrested with an arsenal of weapons and explosives including rocket launchers and Kalashnikov assault rifles in his vehicle, said the SBU.
SBU chief Vasyl Hrytsak said the man had made contact with armed groups in Ukraine with the aim of buying weapons and explosives. In Paris, a French police source said a raid on the suspect's home in France had uncovered ingredients for homemade explosives.
Ukranian authorities said the man was planning to carry out 15 attacks across France targeting bridges, railways, synagogues, mosques, administrating buildings including tax offices and motorways.
"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," Hrytsak told journalists.
The SBU said he was arrested on May 21 while trying to cross into Poland near the Ukrainian frontier town of Yagodyn.
A video posted by Ukrainian security forces purportedly shows the man loading a white van with weapons before he was he was arrested.
A French Foreign Ministry spokesman confirmed the arrest and said the ministry was in contact with the Ukrainian authorities.
The incident added to the security concerns over the staging of European football's showcase event in France after last year's terror attacks in Paris.
French President Francois Hollande on Sunday acknowledged that the threat of an attack during the month-long competition could not be discounted, but promised to "do everything to ensure that the Euro 2016 is a success."
Washington has already warned US citizens about the risk of attacks on stadiums and fan zones.
Around 90,000 security forces will patrol stadiums and crowded areas during the event.
Preparations for the month-long tournament, which begins on Friday, have also been marred by rolling strikes that have paralysed transport and torrential rain that caused flooding along the River Seine and forced tourist attractions including the Louvre Museum to close.