The trial, which has strained relations between Italy and the US, comes as a report just released says the CIA ran secret prisons in Poland and Romania from 2003 to 2005 with the agreement of the countries' governments.
"We believe that the framework for such assistance was developed around Nato authorisations ... some of which are public and some which remain secret"
Council of Europe investigator
Dick Marty, the Council of Europe investigator behind Friday's report, called the prisons in northeastern Poland and southeast Romania part of a "global spider's web" of detention centres and illegal prison transfers.
He also suggested that Nato and the US reached a deal in 2001 allowing the CIA to run the covert prisons.
Marty said in the report: "We believe that the framework for such assistance was developed around Nato authorisations agreed on 4 October 2001, some of which are public and some which remain secret."
The report also said there were "sufficient grounds to declare that the highest state authorities were aware of the CIA's illegal activities on their territories".
Citing interviews conducted with about 30 former and serving intelligence agents in the US and Europe, Marty said the secret prisons were set up by the CIA as a parallel facility to Guantanamo Bay.
The trial of agents alleged to have been involved in the rendition programme, coincides with the expected arrival in Rome of George Bush, the US president.
Italy's government has asked the country's highest court to throw out indictments against the 26 Americans, all but one of them believed to be CIA agents.
The 26 Americans have left Italy, and the US has said they would not be turned over for prosecution even if Rome requests it.
Prodi's government has so far yet to make that request.
The CIA agents are accused of abducting Osama Moustafa Hassan Nasr, also known as Abu Omar and considered a "terror suspect", from a Milan street on February 17, 2003.
Nasr was suspected of recruiting fighters to various organistaions but had not been charged with any crime at the time of his disappearance.
He was taken to US bases in Italy and Germany before being transferred to Egypt, where he was imprisoned for four years.
Nasr, who was released February 11, said he was tortured.
Prosecutors have listed Nasr on a list of more than 120 witnesses, which includes Silvio Berlusconi, Italy's former prime minister in power at the time of Nasr's disappearance, as well as Romano Prodi, the current premier.
The same request was denied by a different judge during the preliminary hearing phase.
Carmelo Scambia, an Italian lawyer for Nasr, said his client wanted to travel to Italy to participate in the trial, despite a pending arrest warrant issued against him by Italian authorities after he was taken to Egypt.
The Italian constitutional court is expected to consider that and another similar appeal in the autumn, and participants in the trial said they expected defense requests to postpone the trial until after the high court rules.
Titta Madia, Pollari's lawyer, said on Friday that he would ask for a suspension of the proceedings pending the constitutional court ruling.
Luciano di Gregori, one of the Italian defendants who worked at the Italian intelligence agency at the time of Nasr's abduction, professed his innocence during a break in Friday's hearing.
"I have been doing this work for 33 years," he said. "I did it with my head held high and in the full light of day. I have nothing to hide."