Saad al-Faqih told Aljazeera.net on Tuesday that his radio stations – that reach more than two-thirds of the Saudi population - had been preparing for the demonstrations for almost seven weeks.
"The Saudi regime is unreformable. This event will involve a series of civil activities whose purpose is clear - to remove the current regime," al-Faqih said.
While privately expecting hundreds of thousands, the London-based exile said the peaceful demonstrations would begin after noon prayers on Thursday - in defiance of an official ban.
Police broke up a much smaller street protest in Riyadh organised by al-Faqih's Movement for Islamic Reform (MIRA) last year to call for the release of political prisoners and greater political participation. Authorities arrested 33 people, but they were later released.
If Thursday's protests go ahead, they may test how much support the MIRA has gathered in Saudi Arabia since dissidents began using radio, satellite TV and the internet to propagate his calls for change from exile.
But al-Faqih says more than 50,000 Saudis are no longer scared to have their names publicly associated with the anti-monarchy movement.
King Fahd and his court are
unreformable, dissidents say
"Influential families, tribes and individuals are now responding to the reform movement - getting the first few to stand up and demand a truly representative government, an independent judiciary etcetera was a challenge.
"But people are now finding safety in numbers and are no longer worried about demanding legitimate change. Every day on our radio stations, Saudis are naming themselves live and adding their voice to the demand for change," he said.
However, a leading Saudi preacher on Monday criticised the plans for demonstrations in the desert kingdom, the world's top oil exporter, saying they were a call for turmoil and subversion.
Shaikh Safar al-Hawali, who was jailed in the 1990s, said al-Faqih's agenda was a pale imitation of Iran's 1979 Islamic Revolution when Ayat Allah Ruh Allah Khomeini toppled the Shah.
"The Saudi regime is unreformable. This event will involve a series of civil activities whose purpose is clear - to remove the current regime"
Saad al-Faqih, demonstration organiser
However, al-Faqih said Hawali was "almost like a prisoner of the regime" and should save his comments until after the demonstrations.
Reformists had originally called for a constitutional monarch with an elected parliament and an independent judiciary, but al-Faqih now says the royal family will not or cannot reform and must go.