A Tunisian opposition party leader who had been critical of the Islamist-led government and violence by radical Muslims has been shot dead, leading to anti-government riots that left downtown Tunis choked with teargas and patrolled by a tank and armoured cars.

Chokri Belaid, leader of the Unified Democratic Nationalist party, was shot outside his home in the capital by a man who fled on a motorcycle on Wednesday morning. He died in hospital shortly afterwards.

The president, Moncef Marzouki, responded by cutting short a visit to France and cancelling a trip to Egypt scheduled for Thursday, and Reuters reported on Wednesday night that he would dissolve the government and try to form a government of national unity.

The killing triggered a 1,000-strong protest outside the interior ministry. Police used teargas to disperse the protesters, who were gathered on the same broad, tree-lined boulevard where two years ago anti-government protests led to the fall of Zine al-Abidine Ben Ali, Tunisia's longtime dictator.

The crowds even chanted the same slogan: "The people want the fall of the regime." As the police moved in people ran for the shelter of nearby buildings yelling: "No to Ennahda," and: "Ghannouchi assassin," referring to the moderate Islamist party and its leader, who dominate the elected government.

Tunis city centre was left deserted and littered with stones, guarded by police armoured vehicles and patrolled by a tank from the national guard. Knots of riot police chased protesters through the elegant streets downtown.

Elsewhere in the country, police responded to a protest in the coastal city of Sousse with teargas while Ennahda offices were attacked in several towns, according to Radio Mosaique and Radio Shems FM.

The prime minister, Hamadi Jebali, who heads the Ennahda-led government elected in 2011 in Tunisia's first post-Arab-spring election, said: "The murder of Belaid is a political assassination and the assassination of the Tunisian revolution. By killing him they wanted to silence his voice."

Belaid was an outspoken critic of the coalition government. His small party co-founded the Popular Front, a leftist alliance preparing to compete in elections this year.

"Chokri Belaid was killed today by four bullets to the head and chest … doctors told us that he has died. This is a sad day for Tunisia," Ziad Lakhader, a leader of the Popular Front, told Reuters.

The man who shot Belaid fled on a motorcycle ridden by an accomplice, according to the interior ministry spokesman Khaled Tarrouch, who called the assassination a "terrorist act" and said the politician had been shot point-blank several times. Police have yet to arrest any suspects.

The reasons for the murder are unclear, but it comes as Tunisia struggles with social and religious tensions since Ben Ali's overthrow in January 2011, which set off revolts across the Arab world.

Belaid had been a fierce critic of Ennahda, claiming it turns a blind eye to violence perpetrated by extremists against other parties. His family said Belaid regularly received death threats – most recently on Tuesday – but had refused to limit his high-profile activities.

His death comes amid reports of intimidation and violence against opposition groups, and days before an official commission of inquiry is due to report its findings on an attack against a trades union rally in December.

The government is in negotiations with opposition parties to reshuffle the cabinet and possibly expand the ruling coalition, but weeks of talks have yielded nothing, as the parties seem unable to reach an agreement over redistributing power.

At the weekend, radicals disrupted a rally led by Belaid in northern Tunisia, one of many political meetings that were disrupted by gangs.

Belaid had been particularly outspoken against the so-called Committees to Protect the Revolution, which many accuse of being behind the violence. These groups are believed to be affiliated with Ennahda and say it is their mission to seek out remnants of the old regime.

"There are groups inside Ennahda inciting violence," Belaid told the Nessma TV channel the night before he was shot. "Rachid Ghannouchi considers the leagues to be the conscience of the nation, so the defence of the authors of violence is clear. All those who oppose Ennahda become the targets of violence."