The unnamed disabled woman had asked to use a toilet at a nearby asylum centre after sharing a taxi with one of its residents.
But after she was invited inside, the woman, in her 30s, was attacked by the man and six of his fellow migrants.
A furious group of more than 100 Swedes have since attacked the centre in Visby, pelting it with rocks.
The victim's lawyer Staffan Fredriksson told local newspaper Aftonbladlet: “She followed him in and had no fears that something would happen.
“Then the man took advantage of the situation. The abuse started in the toilet.