Thousands of people in Mali's capital, Bamako, are flocking to see what it believed to a religious sign on a wall that suddenly appeared last weekend.
Many believe the white image on the outside wall of a toilet shows a man praying, interpreting it as a message from God.
Riot police have been deployed to keep an eye on the crowd as people queue day and night to see the mark.
Most southern Malians are Tijani Muslims, a moderate sect of Sufi Islam.
''We believe it is a vision of our prophet,'' Aliou Traore, who lives in the compound, told the BBC.
''People have come from Senegal to see it and several Malian government ministers and religious leaders have paid us a visit,'' he said.
Mr Traore said the mark has been changing shape since it first appeared.
"Sometimes the white apparition leaves the wall altogether and moves around the compound. Then it goes back,'' he said.
The BBC's Alex Duval Smith in Bamako says people do not have to pay to see the mark but are leaving money in a bucket, which the Traore family say they will give to the local mosque.
When our reporter visited the compound, the mark seemed to look like a drying patch of cement in the shape of a standing woman.
Photos of the image have been circulated widely in Bamako by mobile phone since it appeared on Saturday evening, our correspondent says.
"It's a miracle, I've seen it," schoolteacher Aboubakar Diarra said after looking at the wall.
"It's obviously true. It's a sign from God to Mali that our nation is great."
Followers of the Tijani sect - who are mostly found in West Africa - are known for respecting "miracle" signs.