Judges at the Riyadh court, which was ringed by security forces, issued their verdict on Sunday after a nine-month trial held mainly behind closed doors despite earlier promises of openness.
The court sentenced academics Ali al-Dumaini to nine years in jail, Abd Allah al-Hamed to seven years and Matruk al-Faleh to six years, lawyers said.
All three were arrested in March 2004 after petitioning the kingdom's rulers to move towards a constitutional monarchy and speed up political reforms.
Their arrest - along with nine other men who were later released - drew rare public criticism from the United States which has pushed for reform in its ally since the September 11 attacks, which were carried out by mainly Saudi hijackers.
"This is not fair," Faleh's wife Jamila al-Ukla said after the sentences. Her husband supported "the centrality of the royal family, the country and Islam," she said. "[To call for] constitutional monarchy is not a criminal issue."
Lawyer Ali Ghothami said the men would appeal against the ruling.
Ghothami said the panel of three judges found that the men had "overstepped the bounds" by talking to foreign media, had ignored national interests, intended to incite people and "gave a chance to the nation's enemies to harm it".
They were also convicted of defaming officials and challenging the independence of the judiciary.
"This is not fair ... [To call for] constitutional monarchy is not a criminal issue"
Matruk al-Faleh's wife
Specifically, the judges cited Faleh's criticism of Saudi Arabia's education system, which he blamed for two years of violence by al-Qaida supporters, Ghothami said.
Al-Dumaini had "incited [people] against the Wahhabi school" of Islam in Saudi Arabia, which critics blame for fostering anti-Western sentiment and militancy. Hamed had "challenged the authority of the ruler", according to the court.
Blow to reforms
Relatives and lawyers said they were shocked by the severity of the sentences, but some insisted it was a sign that the authorities realised the reformists were a serious force.
"Today marks the legitimate birth of reform," Hamed's brother Isa said.
The men's arrests provoked rare
criticism from the US, a key ally
A London-based commentator said the verdicts dashed hopes for change.
Fuad Ibrahim, the chief editor of the Saudi Affairs magazine, told Aljazeera: "We were hopeful that the grave mistake committed last year, when Saudi authorities detained some reform figures, would be rectified. But once more, the verdicts proved that there is a persistent tendency to foil any methodology or option to effect reform in the kingdom."
He said the decisions would add to pressure on Crown Prince Abdullah and possibly undermine his credibility.
"If these peaceful means [of dissent] are rejected, this will mean the kingdom is lacking the minimum level of the freedom of expression," he said, suggesting reformists might be compelled to go underground.
Saudi Arabia held partial elections this year to municipal councils - the first national vote in the country. But women were barred from voting, only half the council seats were open to election, and the powers of the councils will be limited.
In urban centres, candidates backed by powerful and conservative religious scholars swept the board.
Most family members spent Sunday's court hearing on a Riyadh pavement after the session was closed off by the judges and security forces insisted the small group of relatives and supporters stay hundreds of metres from the court.
Hamed and Faleh had refused to submit a defence in protest that their court sessions had been held mainly behind closed doors, despite promises last year they would be heard in public.