The head of mobile phone company Vodafone Greece has started giving evidence to a parliamentary committee about a phone-tapping scandal.
The mobile phones of senior officials had been bugged
It is the first time George Koronias has faced public questioning since the scandal came to light.
Last month, the government admitted that the mobiles of the prime minister and other officials had been bugged.
The tapping started before the 2004 Athens Olympics and continued until it was detected last year.
The hearing in parliament is likely to last many hours as Mr Koronias faces a barrage of questions from MPs.
The BBC's Richard Galpin in Athens says the MPs will want to know how it was possible that special software for intercepting phone calls was either secretly activated or secretly inserted within the Vodafone computer system a few months before the Olympics.
This made it possible to bug the phones of at least 100 people, including top politicians, security and military personnel, journalists and Arabs living in Greece.
In his initial comments before the committee, Mr Koronias denied his company was at fault.
MPs are also expected to ask why it took almost a year for the phone-tapping to be discovered.
Speaking just before the hearing began, the Communist MP Liana Kanelli expressed her outrage at the scandal.
"It's one of the greatest this country has ever felt," she said.
She said the material used was "very sophisticated", and that its use pointed to US involvement.
MPs may also ask about the alleged suicide of a top Vodafone network manager, who was found hanged in his house in Athens two days after the tapping was uncovered and a day before Vodafone first informed the government.
The lawyer representing the man's family believes his death is suspicious and has called for his body to be exhumed for further examination.
Vodafone has denied his death is connected to the scandal.
It is widely believed in Greece that US intelligence agents were behind the phone-tapping.
A former American diplomat based at the US embassy in Athens, who resigned three years ago, told the BBC he was convinced the Americans were involved.
He said they did not trust the Greek authorities on the issue of protecting the Athens Olympics from any potential terrorist attack.