TUNIS (Reuters) - A hardline Islamist has been arrested in connection with the killing of a Tunisian opposition politician whose death earlier this month touched off protests across the country, a security source said on Monday.

A woman chants slogans and holds pictures of assassinated leftist politician Chokri Belaid during a demonstration against the Islamist Ennahda movement in Tunis February 23, 2013. REUTERS/Zoubeir Souissi

Tunisia was plunged into political crisis when the secular opposition politician Chokri Belaid was gunned down outside his house on February 6, igniting the biggest street protests since the overthrow of strongman Zine al-Abidine Ben Ali two years ago.

"The police arrested a Salafist suspected of killing Belaid," the source told Reuters without giving more details.

Last year, Salafist groups prevented several concerts and plays from taking place in Tunisian cities, saying they violated Islamic principles. Salafists also ransacked the U.S. Embassy in September, during international protests over an Internet video.

Tunisian radio station Express FM cited a senior security official as saying police had arrested three Salafists, including a police officer, in connection with Belaid's killing.

Abd Majid Belaid, brother of the victim, said he could not confirm or deny the report. The Ministry of Interior and Justice was not available for comment.

Interior Minister Ali Larayedh said last week that arrests had been made but gave no details.

"The investigation has not led yet to identify the killer, those behind the murder and its motives," he said.

Secular groups have accused the Islamist-led government of a lax response to attacks by ultra-orthodox Salafi Islamists on cinemas, theatres, bars and individuals in recent months.

After Belaid's killing - Tunisia's first such political assassination in a decade - Hamadi Jebali resigned as prime minister after he failed to form a cabinet of technocrats to take Tunisia to elections in a bid to restore calm.

Tunisia's President Moncef Marzouki has asked Interior Minister Ali Larayedh to form a new government.

The so-called Jasmine Revolution that toppled Ben Ali in January 2011 was the first of the Arab Spring revolutions.

Tunisia's political transition has been more peaceful than those in other Arab nations such as Egypt, Libya and Syria, but tensions are running high between Islamists elected to power and liberals who fear the loss of hard-won liberties.

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