Thai police has arrested a key member of the nationalistic "yellow-shirts" as he returned from Cambodia on Wednesday's night.

Karun Sai-ngam, a former senator, was put under police custody as he arrived at Suvarnabhumi airport from Phnom Penh, where he joined a legal team in trying to help two other Thais recently sentenced to long jail terms for illegal entry into the neighboring country and espionage.

Karun, belonging to Thailand Patriots Network (TPN), an alliance of the yellow-shirt People's Alliance for Democracy (PAD), was arrested for his role in the seizure of Suvarnabhumi and Don Muang airport by PAD in late 2008.

TPN has spun off from PAD as it has taken a more aggressive approach in the ongoing Thai-Cambodian border disputes, but the two groups remain allied.

Earlier, two other leaders of TPN were also arrested in Bangkok for the same charges, but were later released on bail.