The man, Hussam Taher Hussam, said he had been held in Lebanon by supporters of Saad Hariri, the son of the former prime minister, and subjected to torture and drug injections to force him to testify. Saad Hariri, he said, offered him $1.3 million if he would lie about senior Syrian officials.
Saad Hariri's office issued a statement denying that there had ever been any contact between Mr. Hussam and Mr. Hariri or his associates.
Elie Fawaz, a Lebanese political analyst, said Mr. Hussam's television appearance had been widely mocked in Lebanon as a clumsy attempt by President Assad and his allies to discredit the investigation.
"The image that pops up in my mind is from Maoist China," Mr. Fawaz said. "Mao used to bring people forward and force them to publicly denounce themselves, and that's exactly what's happening now in Syria."