Yuri Savenko, who examined Arap last week together with his colleagues, told RFE/RL's Russian Service that Arap has never posed any threat "to herself or to other people."
Arap claims authorities placed her in the hospital in July in reprisal for a statement she gave to a newspaper, alleging that patients at a psychiatric clinic were beaten and sexually abused.
Arap's daughter, Taisiya Arap, has launched a legal appeal against her mother's detention.
Taisiya Arap told news agencies that police took her mother away while she was collecting documents about her mental health from a doctor in keeping with legal requirements for obtaining a driver's license. She said the doctor called in the police when she realized that Larisa Arap had written the critical newspaper articles.
Arap is the head of the Murmansk branch of the opposition United Civic Front.
Human-rights groups have warned that her case may represent a return to Soviet-era tactics of using psychiatric diagnoses and detentions to suppress opposition.