Former Thai prime minister Thaksin Shinawatra testified in court on Wednesday, seeking to defend himself against royal defamation charges in a watershed case for his faltering political dynasty.

Thaksin faces up to 15 years in prison if he is convicted in the closed-door trial in Bangkok, where he stands accused of breaching strict lese majeste laws that shield Thailand's royal family from abuse and criticism.

For the past quarter-century, the 75-year-old telecoms magnate has been a defining figure of Thai politics, founding a political clan which has jousted with the traditional pro-royal, pro-military elite.