The sentences, issued by Bahrain's military tribunal, call into question talks between the government and opposition that were due to start July 1.
Bahrain's military tribunal on Wednesday sentenced eight Shiite Muslim political dissenters to life imprisonment, and 13 others, including a prominent Sunni critic, to between five and 15 years in jail.
The harsh sentences sparked immediate protests in the villages of the Shiite majority island in the Persian Gulf, which has been ruled by a Sunni Muslim dynasty for more than 200 years.
Those sentenced to life included Hassan Ali Mushaima, who has advocated replacing the monarchy with representative government, and Abdulhadi Abdulla Habil Al Khawajah, a prominent human rights champion.
Ebrahim Sharif, a politician and former candidate for parliament, was sentenced to five years in jail. Mr. Sharif was secretary-general of the moderate secular grouping Waad, or National Democratic Action Society.
Also jailed for 15 years was Mohammed Hassan Jawad, an independent human rights activist who had left his car at the Pearl Roundabout, the epicenter of Bahrain's protests in February and March, and visited Bahrain's police to ask where they'd taken it. He was later charged with conspiracy to topple the Bahrain monarchy.
In the context of at least a dozen other trials now under way, the sentencing of the political dissenters and human rights champions seems unlikely to end the protests on Bahrain, a US ally which is home port to the Navy's Fifth Fleet. Bahrain had a brief flirtation with the Arab spring political reform movement from mid-February until mid-March, when Saudi and United Arab Emirates security forces arrived at the invitation of King Hamad bin Isa al Khalifa.
Already at least 31 people were killed in the violence on the island, and more confrontations now seem likely following the sentencing, putting an enormous question mark over a national dialogue between government and opposition, which is due to begin July 1.