Mr Peeraphan, a key member of the Pheu Thai legal team, complained of dizziness and was rushed to Phyathai Nawamin Hospital, party member Paijit Sreeworakan said.
Mr Peeraphan suffered from ischemic stroke and underwent urgent surgery at the hospital, said Mr Paijit, who is his close friend.
He is safe but doctors were not allowing visitors at this time, he said.
An ischemic stroke occurs when a blood vessel to the brain is blocked by a blood clot. It is distinct from a hemorrhagic stroke that occurs when a blood vessel in the brain becomes weak and bursts open.