A Thai political activist has been sentenced to seven and a half years in prison in the latest in a series of convictions under the kingdom's controversial royal defamation laws.

The Criminal Court in Bangkok found Surachai Danwattananusorn guilty of insulting the monarchy during several public speeches he gave to supporters of his "Red Siam" group in 2008 and 2010.

"This case is political motivated," his lawyer Karom Polpornklang told AFP on Tuesday, adding that he planned to appeal the verdict.

Red Siam is a hardcore offshoot of the Red Shirt movement, which is broadly loyal to fugitive former premier Thaksin Shinawatra, who was ousted by royalist generals in a 2006 coup.

Two months of mass anti-government protests by the Red Shirts in Bangkok in 2010 descended into the kingdom's worst political violence in decades, with more than 90 people killed in a military crackdown.

The royal family is an extremely sensitive subject in Thailand, but calls for reform of the strict lese majeste legislation have grown following several high-profile convictions.

A 61-year-old man was jailed in November for 20 years for sending text messages deemed insulting to the monarchy, while a US citizen in December was handed two-and-a-half years in prison for defaming the king.

Rights groups say the use of the rules to suppress free speech has worsened under the government of Prime Minister Yingluck Shinawatra -- Thaksin's sister -- who rode a wave of support among Red Shirts in an election last year.