Sakoltee Phattiyakul, an anti-government protest co-leader, was taken into custody at Suvarnabhumi airport late on Friday.
Mr Sakoltee will likely face several charges.
Immigration police detained Mr Sakoltee, 37, after he returned from the United States at around 11.30pm on charges of trespassing government offices, obstructing elections and insurrection. More charges will likely to be filed against him.
Officials from the Attorney General's Office, the Royal Thai Police and the Department of Special Investigations investigated him.
DSI chief Tarit Pengdit confirmed on Saturday his detention.
Mr Sakoltee, a former Democrat MP for Bangkok, was later sent to Bangkok's Ratchada Criminal Court where law enforcement officials will seek to detain him without bail.
Their request was based on an arrest warrant of the Pak Kret police station issued on March 25 on the charge of trespassing government offices (Software Park Building). The case was forwarded to the DSI on April 9.
Ekanat Promphan, spokesman of the anti-government People's Democratic Reform Committee (PDRC), told Thai Rath Online that the PDRC had sent a lawyer to represent Mr Sakoltee and seek bail.
Mr Ekanat confirmed the PDRC's stand it would not interfere with evidence.
Mr Ekanat said he personally did not worry much because he had known attempts would be made to file more charges against the PDRC's leaders.
On the "Sakoltee Phattiyakul" Facebook page, the latest post was made at 10.30am on Saturday.
It said: "DSI officials are taking me to the Ratchada Criminal Court for detention on charge of trespassing government offices and several more charges. Thank you for your concern. We'll fight on until we win."
Mr Sakontee is a son of Gen Winai Phattiyakul, a former defence permanent secretary and secretary-general of the Council of National Security, the body set up to rule the country after the 2006 coup.