Human Rights Watch on Monday urged Tunisian authorities to investigate and bring to justice those behind a series of attacks by religious extremists over the past 10 months.

In an open letter to the Interior and Justice ministries, the New York-based group criticized the government for failing to respond to the assaults by extremists against activists, journalists, filmmakers and others.

Victims have described the attackers as bearded religious conservatives known as Salafis, who follow an ultra-conservative form of Islam. They have been increasingly active in Tunisia since the fall of the secular dictatorship a year and a half ago.

"The failure of Tunisian authorities to investigate these attacks entrenches the religious extremists' impunity and may embolden them to commit more violence," said Joe Stork, of Human Rights Watch.

According to the organization, in all the six cases, complaints were filed to police who have yet to arrest anyone or even initiate a formal investigation.

There was no immediate comment from the government as Monday is a holiday.

The letter comes a day after U.S. ambassador to Tunisia Jacob Walles posted a message on the embassy website again urging authorities to bring to justice those behind an attack last month on the embassy.

Several thousand people, many sporting the beards and robes of religious conservatives attacked the embassy, burnt the flag and nearby cars and sacked an American school.

Loosely organized groups have often attacked what they consider symbols of impiety in the country, including film screenings, art exhibits and festivals. Opposition parties have accused the government, which is dominated by the moderate Islamist Ennahda Party, of failing to investigate and prosecute these attacks.

The most recent attack described in the leader was in August against a festival marking the international day for Jerusalem in the city of Bizerte north of Tunis.

Festival organizer Khaled Boujemaa told Human Rights Watch that the festival was repeatedly threatened and then eventually attacked by dozens of bearded assailants, despite several calls to the police. He was badly beaten and ended up in the hospital.

Government officials have repeatedly said that anyone breaking the law would be prosecuted but the attacks have continued, culminating in the Sept. 14 embassy assault.