Political activist Sirawith Seritiwat said on Monday he will seek serious charges against attackers who injured him on Sunday night.
Mr Sirawith, know as "Ja New", said on Facebook that he will ask police to file attempted murder charges against the people who attacked him.
"I will not forgive them," he said. "I never thought they could be so cruel."
The activist was attacked by a group of people on Ratchadaphisek Road in Din Daeng district on Sunday night, and sustained injuries to his head and body. His Facebook Live stream showed bruises on his face.
Mr Sirawith registered the attack with the Huay Kwang police station before he was sent to Police General Hospital. He was later transferred to Mission Hospital by his own request.
The motive for the attack was unknown. He and other campaigners have asked senators not to vote for Prayut Chan-o-cha as the next prime minister. His Facebook message on Sunday also reaffirmed his support for the campaign to "switch off" senators from voting for Gen Prayut.
A joint session of the new parliament will convene on Wednesday to vote for a new prime minister.
Pheu Thai secretary-general Phumtham Wechayachai and Future Forward Party leader Thanathorn Juangroonruangkit posted messages on social media condemning the attack, calling it a "barbarian act".
"This country is difficult to live in" because people with different ideas are not allowed space to express themselves freely, Mr Phumtham wrote on his Facebook page.