Mohammed Fahd al-Qahtani and Abdullah Hamad are founding members of the banned Saudi Civil and Political Rights Association, known as Acpra, that documents human rights abuses and has called for a constitutional monarchy and elections, Reuters reported.
Riyadh, Washington's top Persian Gulf ally, does not allow protests, political parties and trade unions. Most power is wielded by top members of the ruling family and senior clerics of the ultra-conservative Wahhabi school of Sunni Islam.
Last year, Acpra issued a statement demanding that King Abdullah sack his heir and interior minister, Crown Prince Nayef, who they held responsible for rights abuses. Nayef died shortly afterwards.
Qahtani was sentenced to 10 years. Hamad was told he must complete the remaining six years of a previous jail term for his political activities and serve an additional five years.
They will remain in detention until a judge rules on their appeal next month.
Unlike in most previous cases, the trial was opened to the press and public, in what Saudi activists described as a step forward for rights even as they decried the verdict.
Supporters of the two men shouted out that the trial was politically motivated after the judge handed down the sentences, and a line of security officers armed with truncheons cleared the courtroom.
On Thursday, an Interior Ministry spokesman said that activists, whom he did not name, had tried to stir up protests in the world's top oil exporting country by spreading "false information" on social media.
Unrests have hit Saudi Arabia during the Islamic Awakening (Arab Spring) wave of popular uprisings, specially in the Eastern parts of the country mostly populated by the Shiite Muslim minority. But there have also been small demonstrations by Sunni Muslims calling for the release of people held on security charges.
