The first criminal trial over the CIA's "extraordinary rendition" of terror suspects has opened in Italy.
Abu Omar was suspected of recruiting fighters for Islamic groups
Twenty-six Americans and six Italians are accused of kidnapping a Muslim cleric from Italy and sending him to Egypt, where he was allegedly tortured.
The American CIA agents and military personnel will be tried in absentia. Italy has not announced if it will seek their extradition to the Milan trial.
US President George W Bush arrived in Italy hours after the trial began.
Mr Bush will have his first meeting with Pope Benedict XVI at the Vatican on Saturday and will later hold talks with Italy's prime minister, Romano Prodi.
Mr Prodi has already said that the extraordinary rendition case will not be on the agenda.
This is a controversial trial and it made a stuttering start, says the BBC's Christian Fraser in Rome.
In their opening submissions this morning, defence lawyers called for proceedings to go behind closed doors, but the judge rejected their request.
The defence team is supported by the Italian government which has asked the country's highest court to set aside the rendition trial, saying prosecution documents will break state secrecy laws and damage relations with the CIA.
The Constitutional Court is due to rule on that appeal by September, and defence lawyers are expected to ask that the trial be adjourned until the high court makes its ruling.
Prosecutor Armando Spataro told the court that the rendition trial should continue despite the pending decision.
"In a democracy, the separation of powers is critical," he said.
Only one defendant, Luciano di Gregori, who worked for Italian intelligence at the time of the abduction, appeared at the trial.
"I have been doing this work for 33 years," Mr Di Gregori said. "I did it with my head held high and in the full light of day. I have nothing to hide."
The cleric, Hassan Mustafa Osama Nasr - also known as Abu Omar - was snatched from a Milan street in February 2003.
Italian prosecutors say Nasr was taken to US bases in Italy and Germany before being taken to the Egyptian capital of Cairo. Nasr says he was tortured during his four-year imprisonment in Cairo.
At the time of his arrest he was suspected of recruiting fighters for Islamic groups but had not been charged.
He was released by Egypt earlier this year, his lawyer said.
A senior US official has said that the 26 Americans accused of Nasr's kidnapping would not be sent to Italy even if Rome made an extradition request.
Meanwhile, Swiss senator Dick Marty, leading an investigation on behalf of the Council of Europe, said he had evidence that secret CIA prisons "did exist in Europe from 2003 to 2005, in particular in Poland and Romania".
Mr Marty also said a secret agreement among Nato allies provided the framework for CIA activities.