The Bizerte regional branch of the Tunisian League for the Defence of Human Rights had issued a statement denouncing what it called the "blaspheming of the Quran" by prison officials as well as "attacks on the beliefs of political prisoners, and the practice of torture being pursued in Tunisian prisons".
The statement was based on testimony from families of prisoners.
The government issued its own statement strongly denying the allegations.
The four activists from Bizerte, north of Tunis, were held for several hours before being released, league president Mokhtar Trifi said.
One of the activists, Ben Salem, who authored the statement, was to face questioning by judges. Another, Lofti Hajji, heads the Tunisian Journalists' Union, which is not recognised by the government.
Speaking to Aljazeera from Tunis over phone, Hajji said: "It was a kidnapping and not an arrest. I was kidnapped by about 20 security personnel at midday after leaving a conference held by Amnesty International in Tunis.
"The security people took me to the city of Bizerte where I was questioned about the statement that was issued by the Bizerte branch of the Tunisian League for the Defence of Human Rights, a copy of which was sent to Aljazeera on Friday.
Tunisian officials have postponed
a verdict against the rights group
"I was also questioned on what Aljazeera had broadcast concerning the torture of political prisoners in Burj al-Romi jail and the desecration of the holy Quran."
A government official, speaking on the condition of anonymity because of the sensitivity of the issue, told the Associated Pressi that authorities were concerned that the rights group's statement had used references to "sacred Islamic symbols" for inappropriate ends.
Human rights groups have long accused the government of this North African country of torture and holding political prisoners, which the government denies.
Also on Saturday, authorities postponed a verdict in a court case against the group, forcing it to postpone plans to hold a long-awaited conference.
Pro-government members of the league filed suit in September accusing the director of violating internal rules. Citing the lawsuit, authorities blocked the group from holding a forum scheduled that month.
The group tried again to hold the conference May 27-28, but authorities said it had to wait until after the verdict. A ruling is now expected July 8, judicial officials said.