Thai courts have systematically denied bail in lese majeste cases involving red shirt supporters behind the 2009-10 protests, Human Rights Watch said on Sunday.
The New York-based group said bail was denied in 12 cases involving members of the United Front for Democracy against Dictatorship (UDD), while bail was granted for a leader of the rival yellow shirt People's Alliance for Democracy (PAD) in July 2010.
The UDD protested against the government of former prime minister Abhisit Vejjajiva in Bangkok in 2009 and 2010, resulting in street battles that left scores dead and buildings ablaze.
The PAD led protests against a previous government that backed fugitive former prime minister Thaksin Shinawatra, culminating in the seizure of the capital's two airports in 2008.
"Bail appears to be systematically denied to members of the red shirts while they await trial for lese majeste," said Brad Adams, Asia director at Human Rights Watch.
A verdict is expected on Tuesday in the case of Surachai Danwattananusorn, a prominent political activist charged with lese majeste in February 2011. His bail requests have been denied five times.
The lese majeste law was seldom enforced before the September 2006 coup that toppled Thaksin, but there has since been a surge in cases, largely directed at Thaksin's followers, who have become increasingly outspoken in their public criticism of the establishment.
A movement to amend Article 112 of the Criminal Code, the lese majeste law, backed by some prominent royalists who claim that abuses of the law have harmed the institution's reputation, have been rejected by both government and opposition politicians.
"The new government of Prime Minister Yingluck Shinawatra has shown little interest in ending lese majeste crackdowns," Human Rights Watch said. dpa