Ammar Qorabi, head of the National Organisation for Human Rights in Syria, said: "This is the harshest judgment against a prisoner of conscience since President Bashar al-Assad came to power" after his father Hafez al-Assad died.
"Kamal Labwani was sentenced to life, commuted to 12 years in prison, for having contacts with a foreign country aimed at encouraging it to attack Syria."
He said the sentence was all the more surprising because it came amid expectations of an amnesty for prisoners of conscience to mark a planned referendum on the Syrian presidency.
Qorabi also charged that the verdict was "illegal" because it had been modified from the original charges on which the court case was based.
His organisation said Labwani would appeal against the sentence.
The lawyer, Abdel Rahim Ghmaza, said the defendant's wife, sisters, opposition figures and US and European diplomats were in the Damascus court for the verdict.
Rights groups complained last month that Labwani was being held in an underground cell in total darkness and that his health was deteriorating.
He was arrested at Damascus airport as he returned from a tour of foreign countries where he had called for peaceful democratic reform in Syria.
During his time abroad, he held talks with the deputy national security advisor to George Bush, the US president.
Washington in March slammed Damascus for arbitrarily detaining political opponents, and voiced particular concern for Labwani and fellow political prisoner Anwar Bunni who was jailed last month for five years.
Bunni, a prominent human rights lawyer, was arrested in the Syrian capital in May 2006 after signing an appeal for radical reform in relations between Syria and neighbouring Lebanon.
Both court decisions followed a parliamentary election in Syria late last month which was won by the ruling National Progressive Front dominated by Assad's Baath party.
Six leading opposition figures being detained in Syria, including Bunni and Labwani, warned earlier this month that the "repressive climate" in the country was worsening and called for the release of all political prisoners.