Italian spies on Wednesday begin answering charges of abducting an Egyptian imam from a Milan street as part of the CIA's covert programme of transferring terror suspects to countries known to practise torture.
The landmark case involves 25 CIA agents and a US air force colonel along with seven Italian secret service officials including the former head of military intelligence, Nicolo Pollari, who was forced to resign over the 2003 kidnapping.
All the US defendants are being tried in absentia, while the Italian accused were expected to appear in the Ordinary Court of Milan, lead prosecutor Armando Spataro said on Monday.
"I am scheduled to question them, but they have the right to refuse to answer," Spataro told AFP.
The trial will finally get under way after successive Italian governments sought to have it thrown out as a threat to national security.
The issue went before Italy's Constitutional Court, which agreed that part of the investigation had violated state secrecy provisions but said the prosecution could use evidence obtained correctly.
Spataro said the excluded evidence was not crucial to the prosecution's case.
Last week Judge Oscar Magi ruled that Prime Minister Silvio Berlusconi and his predecessor Romani Prodi would not have to testify in the trial, saying their testimony would be "superfluous" and might compromise state secrecy.
The kidnapping took place during staunch US ally Berlusconi's second stint as prime minister, from 2001 to 2006, and he insists that he was never made aware of the operation.
Prodi's subsequent centre-left government followed Berlusconi's policy of refusing to seek the extradition of the 26 US accused in the case, which is among several that have clouded bilateral ties in recent years.
Osama Mustafa Hassan, an imam better known as Abu Omar, was snatched from a Milan street on February 17, 2003, in an operation coordinated by the CIA and Italian military intelligence.
Abu Omar was transferred to a high-security prison outside Cairo, where he was held for four years. After his release in February 2007, he told of torture and humiliation during his incarceration.
His seizure was thought to be among scores of secret abductions around the world since the attacks of September 11, 2001.
Abu Omar, now 47, said he was on his way to his mosque in Milan when he was abducted. He is presumed to have been taken to the US air base in Aviano, northern Italy.
Once in Cairo, Hassan said he was held in a secret service facility where he was submitted to electric charges and manhandling.
He said his interrogation lasted six months until September 14, 2003, according to documents obtained by the Italian press.
Italian prosecutors suspect the cleric of having fought in Afghanistan and being involved in recruiting fighters for jihad, or holy war. Abu Omar has denied the allegations through his lawyer.
Spataro is known for his work against the left-wing militant group the Red Brigades that was active in the 1970s.