The court said its warrant for Mr Sombat's arrest was only intended to allow authorities to bring him to the court to press charges and detention was not required, according to Viroj Phumsirisawasdi, a lawyer for the People's Democratic Reform Committee (PDRC).
The DSI said it was waiting to process Mr Sombat's fingerprints andargued that detention would prevent him from instigating further anti-government demonstrators.
DSI officials arrested Mr Sombat at the National Institute of Development Administration (Nida) in Bang Kapi district when he arrived there alone. Mr Sombat is the director of Nida's Asean and Asia Studies Center and a former rector of the institute.
He is among the 30 PDRC leaders who are charged by the government with offences including insurrection and face arrest warrants that the Criminal Court issued on Wednesday.
Mr Sombat said he and other academics supporting the PDRC had planned to surrender to police next week. In any case, he said, they had the constitutional right to demonstrate and intended to educate the public about politics through the PDRC demonstrations. He denied all charges levelled by the government.
There was also a report that police arrested Yossak Kosaiyakanont, another PDRC co-leader facing the same charges as Mr Sombat, near his house on Lat Phrao 91 Road in Bangkok on Friday.