According to the Paris-based Tunisian Solidarity Movement (TSM), Muhammad Abu was detained on Wednesday after an article he wrote was published on the internet.
Abu is a member of the Tunisian Committee of Young lawyers, the International Association of Support for Political Prisoners, and other justice groups.
"Abu criticised President Bin Ali's decision to invite the war criminal Israeli Prime Minister Ariel Sharon to take part in the World Summit on the Information Society which will take place in Tunisia on 16-18 November," the TSM said in a statement.
"The Tunisian Solidarity Movement expresses its solidarity with Muhammad Abu and demands his immediate and unconditional release."
"It reminds the Tunisian authorities of their human rights and freedom of expression obligations which they have continually contravened."
Tunisian authorities were unavailable for comment on Abu's arrest.
Israeli PM Sharon has accepted
the invitation to visit Tunisia
Tunisia is North Africa's most stable and prosperous country, but human rights groups have repeatedly accused the government of muzzling the press, beating dissidents and restricting political life.
A report issued by the The International Freedom of Expression Exchange last week warned that freedom of expression is under siege in Tunisia.
The group said human rights activists are being intimidated, dissident internet sites closed down, and books and newspapers censored.
Freedom of expression
The report said there is extensive police surveillance of email messages and internet cafes and that individuals are detained for expressing their opinions.
However, Tunisia says freedom of expression is a reality in the country and that opposition members express their views freely and without any restrictions.
"[The TSM] reminds the Tunisian authorities of their human rights and freedom of expression obligations which they have continually contravened"
Tunisian Solidarity Movement
"Opposition newspapers publish regularly and express themselves on all issues of the day. The media are free in Tunisia and there is absolutely no press censorship. Media freedoms are protected by law and in practice," a government statement last week said.
Tunisia broke off diplomatic ties with Israel after the outbreak of the Palestinian uprising in September 2000, although some commercial relations continue.