A member of Bahrain's largest Shiite opposition grouping, the Islamic National Accord Association (Al-Wefaq), slammed the sentence as contradicting King Hamad's calls for dialogue, set to begin on July 1.
"Is this the atmosphere for dialogue?" asked Khalil Marzooq in excerpts of a speech he gave at a press conference in Manama posted on his Facebook page.
"When the one calling for change and reform is sentenced to life in prison, how will others take part?" he asked.
"There are political forces, some of whom have received harsh sentences today, which have not been invited for dialogue," he added. "How will there be a dialogue without those figures?"
The eight activists sentenced to life include Hassan Mashaima, head of the Shiite opposition Haq movement, and Abdulwahab Hussein, who leads the Shiite Wafa Islamic Movement, as well as Shiite human rights activist Abdulhadi al-Khawaja, who is also a Danish citizen.
Activist and Haq member Abduljalil al-Singace, who was released in February after six months in jail, was also sentenced to life.
The other four are Mohammed Habib al-Muqdad, who holds a Swedish passport; his cousin Abduljalil al-Muqdad and Saeed Mirza, both of whom are Wafa members; and Said Abdulnabi Shihab, who was sentenced in absentia.
Ibrahim Sharif, the Sunni leader of the Waed secular group, who played a prominent role in month-long protests for democratic reform that were crushed in March, received a five-year sentence, BNA said.
Washington expressed concern about the use of military courts to try the activists.
"We are concerned about the severity of the sentences handed down... in Bahrain. We're also concerned about the use of military courts to try these civilians," State Department deputy spokesman Mark Toner told reporters.
"Such steps are at odds with the universal rights of Bahrain's citizens," he added.
Sharif and other leading opposition figures were arrested amid the Sunni authorities' crackdown on the protests led by the islands' Shiite majority.
Nine of the defendants had been in custody on similar charges in the past before being set free under a royal pardon in February aimed at calming the protests.
Mashaima, who was abroad, returned to Bahrain following the pardon. His son Ali Hassan Mashaima, who is in London, is being tried in absentia and was sentenced with five others to 15 years.
The sentences sparked protests in several Shiite-populated villages, activists told AFP.
Scores more activists are facing trial on charges linked to the protests in a semi-martial court set up under a "state of national safety" decreed by King Hamad a day before protesters were evicted from a Manama square in mid-March.
Authorities backed by troops that rolled into Bahrain from fellow Gulf nations quelled the protests while security forces set about arresting hundreds of activists, as well as doctors, medics and teachers accused of backing protesters.
Bahrain's interior ministry said 24 people, including four policemen, were killed in the unrest. The opposition said scores were arrested, amid widespread allegations of torture, while hundreds were dismissed from their jobs.
Four people have been sentenced to death and three others to life in prison over the killing of two policemen during the protests. Nine others were jailed for 20 years after being convicted of abducting a policeman.