PAD supporters occupied Bangkok's main airport last year
Thai Foreign Minister Kasit Piromya has refused to resign after police questioned his involvement in last year's Bangkok airport siege.
Mr Kasit said he would fight his case in court.
Mr Kasit was among more than 30 leaders of the People's Alliance for Democracy (PAD) summoned by police this week.
Persistent protests by the PAD, also known as the Yellow Shirts, contributed to the ousting of a government loyal to the deposed Thai PM Thaksin Shinawatra.
Thaksin supporters complained when Mr Kasit was appointed foreign minister by new Prime Minsiter Abhisit Vejajiva.
Mr Abhisit says he will not force Mr Kasit to quit the foreign ministry post.
His Democrat Party said it would allow Mr Kasit to exercise his own judgement on the matter, and would only decide on his movements if Mr Kasit is formally indicted and his case filed in court.
Other PAD leaders called to the police for questioning include media mogul Sondhi Limthongkul, retired general Chamlong Srimuang and Somsak Kosaisuk.
The Nation newspaper reported that Mr Kasit faced terrorism charges.
The opposition Pheua Thai Party, backed by Mr Thaksin, said Mr Kasit should step down immediately in the national interest.
Mr Kasit was seen speaking at several rallies by the Yellow Shirt protesters, who occupied the prime minister's office compound for three months and shut down Bangkok's airports.
Mr Kasit later described the blockade - which left mroe than 250,000 travellers stranded - as an "innovation in public protests".
Since Mr Abhisit's government took control in December, there have been several street protests by Mr Thaksin's supporters, known as the Red Shirts. Rioting in April left two dead and more than 120 injured.