An elderly nun was found beaten to death in an east-end Montreal convent Monday morning, and the prime suspect is a troubled man she had been trying to help.

Sister Estel Lauzon, 81, belonged to the religious group Sisters of Providence.

Her body was found in the convent's outreach centre, where she would likely have been trying to help the 31-year-old suspect.

"We know that the man probably has a history of psychiatric problems. He was arrested and is presently being interrogated by our police officers," said police spokeswoman Lynne Labelle.

"We know there was no firearm used. It's probably physical force, but we're not sure at this time if an object was used to hurt her."

Favio Paz, who worked with Sister Estel in the outreach centre, said she "was a nice lady who prayed and looked after everyone."

Police have not released the name of the suspect.