
Prime Minister Samak Sundaravej said he had ordered police to take swift action against the activist.
"I instructed police to immediately charge the rally speaker after receiving a report on offending remarks involving many institutions," Samak said.
Police apprehended anti-coup activist Daranee Chanchoengsila-pakul - also known as Da Torpedo - and charged her with offending the monarchy.
Da Torpedo is a reporter who turned activist following the 2006 coup. She has been a regular speaker at anti-coup rallies. On Friday she made a speech opposing the People's Alliance for Democracy.
She is known to be a staunch opponent of the PAD and has attended many counter-rallies at Sanam Luang and at various PAD sites, including the office of television station ASTV.
Metropolitan Police commissioner Lt-General Aswin Kwanmuang said the arrest was based on recorded remarks by Da Torpedo.
Da Torpedo accused police of double standards, saying they had refused to act against speakers at PAD-led rallies.
She said she would not give a statement to police and reserved her right to testify only in court.
She said police had overstepped the mark by arresting her before summoning her to report to them.
Aswin said police had acted within the proscribed procedures. Any serious offences punishable by more than three years in jail give police the leeway to bypass a summons for questioning, he said.
If convicted, Da Torpedo faces three to 15 years in jail.