Mr Sonthiyan was seized by police on Monday afternoon while eating at the Central Department Store at Lat Phrao. He and 18 other PDRC leaders are wanted under warrants for violating the emergency decree in connection with the PDRC protests demanding the resignation of caretaker Prime Minister Yingluck Shinawatra and her government.
The police showed up at Mr Sonthiyan's house on Phutthamonthan Sai 4 road about 6am, but found no evidence related to the case, reports said.
However, they did apprehend a man believed to a relative of Mr Sonthiyan and charged him with having a war weapon in his possession.
Mr Sonthiyan is being detained at the Region 1 Border Patrol Police headquarters in Pathum Thani province for interrogation.
Pol Maj Yuthana Praedam, a special cases specialist of the Department of Special Investigtion (DSI), said that under the emergency decree the police are empowered to detain him for questioning for seven days without bail.
After the first seven days, the police are required to seek approval from the Criminal Court to detain him for another seven days at a time, but no longer than 30 days in total.
At each court appearance the suspect can submit a request for bail. If the request is rejected, he remains in police custody.