Is prayer valid in mosques that contain graves?

Mosques that contain graves should not be prayed in. The graves must be exhumed and their remains transferred to public cemeteries, because the Messenger of Allah, peace and blessings be upon him, forbade that and cursed the Jews and Christians for taking the graves of their prophets as places of worship. He said: "Those are the people who, when a righteous man among them dies, build a place of worship over his grave and make those images in it. Those are the worst of creation in the sight of Allah."

It is obligatory to keep graves away from mosques, in compliance with the command of the Messenger of Allah, peace and blessings be upon him, and as a caution against the curse and falling into polytheism. If the graves are old and a mosque was built over them, then it is obligatory to demolish and remove it.