Jatuporn Prompan, chairman of the United Front for Democracy against Dictatorship, was sent to jail for breach of his bail conditions by the Criminal Court on Tuesday.
The court revoked bail earlier allowed him on charges of terrorism linked to the violent red-shirt street protests in 2010.
Prosecutors handling special cases for the Criminal Court sought revocation of bail for five UDD leaders in the terrorism case -- Mr Jatuporn, Veerakant Musikaphong, Nattawut Saikuar, Weng Tojirakarn and Nisit Sinthuprai.
The five are among 24 defendants in the terrorism case, filed against them in 2010.
Prosecutors said the five defendants had in 2015 made remarks on several television programmes which could be construed as stirring unrest and violating the rights of individuals or state agencies, in violation of the conditions set for their temporary freedom on bail.
All five appeared before the court on Tuesday morning to hear the ruling. A number of red-shirt supporters were present at the court in a show of support.
The court ruled the evidence showed that only Mr Jatuporn, the second defendant, had broken the bail conditions. The other four defendants had only voiced their opinions, without breaching the conditions.
The court statement said Jatuporn had breached bail conditions by being "sarcastic" about the work of the junta, AFP reported.
"The suspect went on a television programme and implicated other people and caused damage to their dignity and reputation with harsh words," the statement said.
After the court's ruling, Mr Jatuporn was taken straight to the Bangkok Remand Prison, where he was detained.
Speaking to reporters before the hearing Mr Jatuporn greeted the prospect of prison with dark humour.
"I have been imprisoned twice so if today the court revokes my bail and sends me to prison I have prepared my personal stuff, although I have only brought my sneakers, not socks," he said.
The remark was a reference to the recent death of a suspect in Department of Special Investigation custody who reportedly hanged himself using a sock.
Kobkiat Kasiwiwat, director-general of the Corrections Department, said Mr Jatuporn would not be put in a cell containing detainees suspected of being his opponents, for his safety.
Mr Jatuporn would undergo a physical examination before being put in detention. He would get the same treatment as other detainees, he added.
UDD lawyer Winyat Chartmontree said he would submit a new application for bail for Mr Jatuporn in a week or two.