Bahrain, a US ally against Iran and home to the US Fifth Fleet, has been in political ferment since a street revolt led by majority Shiite Muslims erupted in early 2011, part of a tide of unrest against autocratic rulers across the Arab world.
On Monday, Bahrain's highest appeals court upheld sentences ranging from five years in prison to life that were originally handed down by a military court in June 2011 to protest leaders, a ruling that could kindle further unrest.
Bahrain's case has stirred international criticism, with US officials calling for acquittals to help restore calm and stability in the island monarchy.
New York-based Human Rights Watch said its own investigation showed that evidence against the convicted men was based on public statements in which they called only for reforms and on confessions apparently obtained through coercion.
"The mind-boggling verdicts in these cases did not mention a single recognizable criminal offense, instead pointing to speeches the defendants made, meetings they attended, and their calls for peaceful street protests in February and March 2011," said Joe Stork, HRW's deputy Middle East director.
"Bahrain's Cassation Court has proven its inability to protect the most basic rights guaranteed in Bahrain's constitution and the international treaties it has signed," he said in a statement.
The government says Bahrain's courts are independent, though the Sunni Muslim ruling Al Khalifa family holds senior cabinet portfolios including justice and key posts in the judiciary.
The Court of Cassation is headed by an Al Khalifa relative and King Hamad heads the Supreme Judicial Council.'Toeing the government line'
Amnesty International said the ruling highlighted "flaws" in the justice system and the defendants should be released.
"This unjust decision will confirm the view of many that the judiciary is more concerned about toeing the government's line than upholding the rule of law and the rights of all Bahrainis," said Hassiba Hadj Sahraoui, deputy director of London-based Amnesty's Middle East and North Africa program.
The main charges the protest leaders faced were "forming a terrorist group with intent to overthrow the system of government", as well as collaboration with a foreign state - an allusion to Iran which Bahrain says fomented the unrest.
The men denied all charges, saying they wanted only democratic reform. They also deny links to Iran over the protests, and Tehran for its part has denied involvement.
The government, backed by troops from Saudi Arabia and police from the United Arab Emirates, put down the uprising with martial law in March 2011. Thousands were arrested and military trials conducted, and an investigation by international legal experts said systematic torture had been used against detainees.
Bahrain says it is now instituting a range of political, legal, police, education, media and other reforms.
But opposition leaders have dismissed those plans as cosmetic and protests have continued, albeit on a reduced scale.
Police and demonstrators clash almost daily and each side blames the other for the violence and the absence of talks to end the conflict.
DUBAI (Reuters) -- An appeals court ruling confirming prison terms for 13 protest leaders in Bahrain this week shows that the Gulf Arab state's judicial system is flawed and unable to protect basic rights, international activist groups said.