Amnesty International says a ban on the construction of minarets would breach Switzerland's obligations to uphold freedom of religion.
Swiss voters will decide on Sunday on a rightwing proposal to forbid the building of the Islamic structures.
"A ban on the construction of minarets while, for example, allowing those of church spires would constitute discrimination on the basis of religion," said Nicola Duckworth, Europe and Central Asia Programme Director at Amnesty International.
"Contrary to the claims of the initiators of the referendum, a general prohibition would violate the right of Muslims in Switzerland to manifest their religion," Duckworth said.
The proposed ban was launched by members of the rightwing Swiss People's Party and a small religious party.
In the latest opinion poll, 53 per cent of respondents said they would reject the initiative, while 37 per cent said they would approve the ban.