A riot broke out after police tried to disperse a crowd gathered to watch the rumoured hanging of a "woman-tigress", Iran's Jomhuri-Eslami paper reports.
Some 50 people were arrested after the would-be spectators turned on police who tried to disperse them in the city of Qom, said the hardline daily.
It was rumoured a woman had "insulted religious values" during Ramadan.
According to local folklore, anyone who commits such a crime risks having their head turned into that of an animal.
A local journalist told French news agency AFP that rumours had circulated of a woman insulting Islamic values during the fasting month.
Her head had been turned into that of a ferocious female tiger as divine punishment, it was rumoured.
"Drawings of the woman with the head of a tigress were even distributed in the city, especially in schools," said the journalist, who asked not to be named.
"The rumour was that she was to be hanged."
A large crowd turned up in a square in the city to watch the hanging - only to be met by police attempting to convince them the rumour was false.
In the ensuing violence, several windows of surrounding buildings were smashed, said the newspaper.