Police fired tear gas Sunday at protesters as opponents and supporters of Tunisia's Islamist-led government clashed outside parliament after the burial of the second opposition figure slain this year.

Mohamed Brahmi's cold-blooded murder on Thursday outside his home has stoked tensions in the North African nation where the Arab Spring began.

Many Tunisians blame the government for not reining in radical Islamists accused of a wave of attacks since strongman Zine El Abidine Ben Ali was toppled in a popular uprising in 2011.

Opposition figures are calling for the government to resign and the powerful General Union of Tunisian Labour (UGTT) was due to convene Monday "to decide the fate" of the country, its secretary general Sami Tahri said.

Radical Salafists close to the Al-Qaeda-linked group Ansar al-Sharia, blamed by the authorities for Brahmi's murder, denied any involvement in an online statement Sunday.

Brahmi was gunned down in the Ariana suburb of Tunis, his body riddled with 14 bullets, almost six months after the murder of opposition politician Chokri Belaid.

Authorities say the same gun was used in both killings, and blamed jihadists close to Ansar al-Sharia.

But in a Facebook statement, the group denied responsibility for what it called "a political assassination, part of attempts to push the country toward chaos."

Brahmi's murder "only profits remnants of the former regime and lackeys of the Zionists and Crusaders," it said.

Hundreds of thousands of mourners thronged the streets of Tunis on Saturday in an emotionally charged funeral to El-Jellaz cemetery where Brahmi was buried next to Belaid.

Slogans vowing to "avenge" Brahmi and Belaid rose from the sea of mourners.

After the burial, protesters calling for the fall of the government marched on the Constituent Assembly and clashed with riot police who fired tear-gas to disperse them, an AFP reporter said.

An opposition MP was injured by a blow to the head.

The demonstrations later tapered off but erupted again overnight when thousands of supporters and opponents of the government led by the Ennahda party squared off outside.

An AFP reporter said rival protesters camped outside parliament until dawn, separated by security barricades and anti-riot police.

"Enough with Ghannouchi," the opposition crowd chanted, referring to Ennahda chief Rached Ghannouchi. "The people want the fall of the assassins."

Ennahda supporters retorted that the parliament was a "legitimate" body and warned there was no room in Tunisia for the likes of Egyptian armed forces chief General Abdel Fattah al-Sisi.

Sisi led a coup that toppled elected Islamist president Mohamed Morsi in Egypt on July 3.

Supporters of the deposed president have camped out in Cairo for a month demanding his reinstatement, and some 300 people have been killed in violence since the coup.

In Tunis before dawn, police fired tear gas when protesters began hurling rocks at each other. Security forces also dismantled tents that anti-government protesters had erected outside parliament.

Interior Minister Lotfi Ben Jeddou on Sunday pledged to guarantee the safety of anti-government demonstrators, leftist MP Samir Taieb said.

"The minister told us that he has clearly given orders for (security) agents not to use force against demonstrators and those who take part in the sit-in before the National Constituent Assembly," he said after a meeting with Ben Jeddou.

Taieb earlier told AFP that the government should step down and hand over to an administration of national unity.

The number of MPs who have decided to boycott the Constituent Assembly since Brahmi's murder has risen to 65 -- nearly a third of the 217-seat strong parliament, according to Taieb.

He warned that Tunisia was on the brink of "unprecedented (security) breakdown if the government does not resign."

Parliament's speaker Mustapha Ben Jaafar, meanwhile, called for "restraint" from his assembly colleagues and urged MPs to work to reach a so far elusive agreement on a new constitution by the end of August.