Larisa Arap says she is being forcibly held in a psychiatric clinic because she criticised health officials.
The ombudsman has been asked to intervene by the United Civil Front (UCF) opposition group, of which Ms Arap is a member.
The Russian government has not commented on Ms Arap's situation.
A spokeswoman for the Russian human rights ombudsman told Reuters agency an independent psychiatric commission would be sent to examine Ms Arap at the hospital where she is being kept.
Chilling.