The group includes a rights activist who staged a 110-day hunger strike last year to protest the verdicts, which critics have denounced as part of attempts to crush an uprising began nearly two years ago in the island kingdom.
More than 55 people have died in the unrest since February 2011 and many opposition leaders and activists have been arrested, including the group of 20 charged with “plotting to overthrow” the regime by leading the protests.
Defense attorney Jalil al-Aradi said the high court refused to reconsider the sentences or convictions, which were originally handed down in 2011 by a military-led tribunal created under temporary martial law-style rules. The group has said they faced abuses while in custody.
Among the eight sentenced to life is rights activist Abdulhadi al-Khawaja, who staged a more than three-month hunger strike last year in protest. The other 12 have sentences ranging from five to 15 years, with seven convicted in absentia.
The case has brought international pressure on Bahrain, including efforts by Denmark to free al-Khawaja, who also holds Danish citizenship.
Scattered protests broke out in Bahrain shortly after the court decision, which could close all further appeal options.
Bahrain's main political group, Al Wefaq, said the courts are used as a “tool to punish the opposition” and suggests that authorities are not serious about offers for a political dialogue to ease the tensions.
“It seemed there is no solution for the Bahraini crisis,” said a statement by the group, “because there is no serious desire to solve this crisis.”