Passi was admitted to a hospital in Haldwani after he took ill this morning, police said.
Doctors attending to him said he had become very weak and his blood pressure had shot up.
Passi was arrested on charges of violating prohibitory orders and taking part in a demonstration demanding withdrawal of a case against BJP MLA Rajkumar Thukral in connection with 2011 Rudrapur riots.
Passi has refused to obtain bail and is demanding his unconditional release.