"I don't know what happened, my friend, but whoever is doing this has to be stopped," said the leader of the mosque, Imam Husham al-Husainy.
The vandalism, in dripping red paint, was discovered at about noon and reported to police, according to Husainy and Dawud Walid, executive director of the local chapter of the Council on American Islamic Relations.
"The mosque has been vandalized so many times, they've basically given up on law enforcement, given up on telling anyone," Walid said.
Dearborn police declined comment. Two Dearborn squad cars were parked outside the center Wednesday night as officers took a police report.
Worshipers gathered Wednesday night after the incident, worried that the violence could escalate.
"Now we worry about our imam," said Ali Edriss of Dearborn, a member of the center. "We worry someone might hit him or kill him. Maybe next time they'll shoot someone. It's scary."