
It was unclear how many gunmen were involved.
Police initially said that a group of five heavily-armed men broke into Al Pukon Mosque in Joh I Rong district through the back door and sprayed the bullets on about 50 people inside the mosque.
Police said ten of them, including an Imam, died instantly at the scene.
About 12 others were injured.
However, later reports said only two gunmen carried out the attack.
The attack on praying Thai-Muslims was the first of its kind and would deal a major blow on the Abhisit government's efforts to bring peace back to the deep South.
Latest reports also said one of those injured has died, bringing the death toll to 11.
A manhunt has been launched for the attackers. The mosque and surrounding areas have also been sealed off, TV reports said.