Mario Scaramella was arrested in Naples after returning from London. He has been accused of international arms trafficking and slander, and he was being taken to Rome, according to his father, Amedeo Scaramella.
Scaramella is the first person connected to the poisoning case to be arrested.
Scaramella met Alexander Litvinenko at a London sushi bar on November 1, the day the former KGB agent fell ill. Litvinenko died of poisoning from radioactive polonium-210 on November 23.
Güncelleme Tarihi: 20 Eylül 2018, 18:16
Scaramella said he showed Litvinenko emails from a confidential source identifying the possible killers of Russian investigative journalist Anna Politkovskaya and listing other potential targets for assassination - including himself and Litvinenko.
On his deathbed, Litvinenko blamed Vladimir Putin, the Russian president, for involvement in his poisoning - an allegation that the Kremlin denied.
British police had no comment on Scaramella's arrest. However, the charges against him were not believed to be directly related to the poisoning case.
Scaramella also was hospitalised for several days in London and he said doctors told him he had received five times the lethal dose of polonium-210, although he showed no symptoms. He left the hospital a few days later.