Thousands of Tunisians have gathered to protest over the assassination of leading opposition politician Mohamed Brahmi, Sky News reported.

Brahmi was shot dead by gunmen outside his home on Thursday, July 25.

The protesters massed in front of the interior ministry building in Tunis and called for the government's resignation, shouting slogans such as "Down with the rule of the Islamists!"

Large protests have also been reported in Sidi Bouzid, the birthplace of the Tunisian revolution, where he was MP.

Brahmi, 58, was a member of the nationalist and secular Popular Party. He was also one of the politicians helping to write a new constitution for the North African country.

His killers reportedly fired at his car before speeding off on a motorbike.

It is the second assassination of an opposition politician following the murder of another leftist politician, Chokri Belaid, in February. That assassination led to a political crisis, mass protests, and the resignation of Islamist Prime Minister Hamadi Jebali.

The government has blamed Belaid's assassination on Islamic extremists and said six suspects will soon be identified.

Tunisia, the birthplace of the Arab Spring, is struggling with a democratic transition after the overthrow of dictator Zine El Abidine Ben Ali in 2011. The country is now led by the moderate Islamist Ennahda Party, which rules in a coalition with two secular parties.

However, the opposition has criticized Ennahda for not cracking down on hardline Islamists, who have been blamed for many acts of violence in the last few years.

Ennahda has condemned the latest killing as a "cowardly and despicable crime".

In a statement, it called on the government to "urgently arrest those who committed this crime and reveal those behind them who have targeted the stability of the country".

A day of mourning has been declared for Friday.