The National Council for Rights has condemned the arrest of activists who were seen carrying banners calling for a "no" vote on the draft constitution.
Egypt will vote on a draft constitution on Tuesday and Wednesday in a referendum seen by the interim government as a stepping stone towards democracy, but by Islamists, who will boycott it, as illegitimate.
The council said that it is against the arrests as long as the activists were expressing their opinions through peaceful means, adding that building a democratic state must begin with respecting the people's rights to freely express themselves.
The Strong Egypt Party, who has called for the boycotting of the vote, had announced that a number of its members were arrested for hanging banners which called for a "no" vote in the referendum.
This party was founded by Abdel Moneim Aboul Fotouh, a former Muslim Brotherhood member and a defeated presidential candidate.
The council asked the authorities to release the activists immediately.
Egypt's army has promised intense security of the voting process in cooperation with the police.
The draft, if passed, will replace a constitution written by an Islamist-led assembly in 2012 under the Mohamed Mursi administration.
The army ousted Mursi in July in response to mass demonstrations against his rule, suspended his constitution and called for early parliamentary and presidential elections.
Elections are expected to kick off after the referendum.