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, 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.