The American Freedom Defense Initiative went to court after the Southeastern Michigan Transportation Authority refused to accept the ads, The Detroit News reported. The initiative is run by Pamela Geller, who also heads Stop Islamization of America and is one of the most vocal opponents of the so-called ground-zero mosque in New York.
U.S. District Judge Denise Hood granted Geller's group a preliminary injunction Thursday.
"This is a huge win, not just for us, but for the First Amendment," Geller said on the initiative's Web site.
The ads, to be run on the side of buses, say: "Fatwa on your head? Leaving Islam? Refuge from Islam.com. Got questions? Get answers!"
Victor Begg, an adviser to the Council of Islamic Relations of Michigan, said Muslims who want to leave the faith are free to do so. He called the ads "another example of a hate group taking advantage of the First Amendment."