Dozens of plainclothes policemen blocked roads leading to the headquarters of the Tunisian Human Rights League and allowed no one to approach the site, the witnesses said on Saturday.
Security officials traded insults with members of the league while European and US diplomats and French lawyers, who were invited to the congress, looked on.
Witnesses said police beat and kicked league activists who tried to breach the cordon.
No arrests were reported.
The incident was the latest in a series of what rights activists say are government abuses, including beating lawyers, jailing opponents and stifling the press.
Mokhtar Trifi, the president of the group, said: "It is shameful that those invited to support human rights in Tunisia cannot gain access to the headquarters of the human rights league."
Some pro-government members of the league say Trifi and other leaders have muzzled a free debate before the planned meeting.
"It is shameful that those invited to support human rights in Tunisia cannot gain access to the headquarters of the human rights league"
Tunisian Human Rights League
They accuse the league's leadership of cronyism, physical and verbal violence against militants and sidelining members who disagree with them.
As a result of their complaints, a court ruled that the meeting would be illegal pending further judicial hearings next month.
Meanwhile, Aljazeera reports that members of the Tunisian National Organisation for Lawyers have begun a hunger strike in protest against the law of the Supreme Bar Institute which was endorsed by the Tunisian parliament.
The lawyers accused the ministry of justice of seeking to control the lawyers syndicate.
Rights activists say Western governments tend to focus on Tunisia's strong economic performance and the government's success in tackling Muslim extremists while turning a blind eye to what they say is often heavy-handed tactics to suppress dissent.
The government says it is committed to democracy and respect of human rights, adding it has no political prisoners and that no one has been jailed for expressing their opinions.