The killing of Chokri Belaid, a 47-year-old lawyer, heightens tensions in the North African nation whose path from dictatorship to democracy has been seen as a model for the Arab world so far.

Police used tear gas to disperse the thousands of protesters that assembled in front of the Interior Ministry in the center of the capital to accuse the government of allowing the assassination to happen. At one point an ambulance carrying Belaid's body was driven in front of the ministry accompanied by protesters before they too were driven off.

The demonstrators were gathered on the same broad, tree-lined boulevard where weeks of anti-government protests two years ago ousted Tunisia's long-time dictator — and the crowds Wednesday even chanted the same slogan: "The people want the fall of the regime!"

Like two years ago, police soon resorted to tear gas, sending people running 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.

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

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

Belaid, a leading member of a leftist alliance of parties known as the Popular Front, was shot as he left his house in the capital, Tunis, and was taken to a nearby medical clinic, where he died, the state news agency TAP reported. Interior Ministry spokesman Khaled Tarrouch called the assassination a "terrorist act" and said the politician had been shot point-blank several times.

The motive behind his killing is unclear. It comes as Tunisia is struggling to maintain stability and revive its economy. The revolution set off revolts across the Arab world and unleashed new social and religious tensions in the Mediterranean nation of 10 million.

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

Tunisian President Moncef Marzouki, a member of a secular party in the governing coalition, called the assassination a threat against all Tunisians.

"Chokri Belaid was murdered this very day knowing I was going to be speaking to you," he told the European Parliament in Strasbourg, France. "This is a letter being sent to us that we will refuse to open. We reject that message and we will continue to unmask the enemies of the revolution."

Afterward in a news conference, Marzouki said the assassination would not derail the country's transition to democracy.

"All these destabilization attempts — and there will be others because for some the Tunisian model should not succeed — I can tell you that we will face the challenge and defeat it," he said.

EU Parliament members held a moment of silence for Belaid.

Marzouki headed straight back to Tunis after speaking, having canceled a trip to Cairo to attend the Organization of the Islamic Conference.

Ennahda leader Rachid Ghannouchi told The Associated Press the slaying was an "ignoble crime" and urged authorities to bring the perpetrators to justice. He also offered his condolences to Belaid's family and followers and called for a day of mourning.

Ghannouchi said those behind the slaying are "parties whose interests are threatened by the revolution and the democratic transition."

Tunisia's Islamist-led government is in negotiations with opposition parties to reshuffle the Cabinet and possibly expand the ruling coalition. Weeks of talks have yielded nothing, however, as the parties seem unable to reach an agreement over redistributing power.

Over the weekend, radicals disrupted a rally led by Belaid in northern Tunisia, part of a string of 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 the Ennahda Party 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 defense of the authors of violence is clear. All those who oppose Ennahda become the targets of violence."

French President Francois Hollande condemned the killing and expressed worry over the rise of political violence in Tunisia.

"This murder deprives Tunisia of one of its most courageous and free voices," Hollande said in a statement. "Belaid was committed throughout his political life to the fight for freedom, tolerance and the respect for human rights."

Associated Press writer Raf Casert in Strasbourg contributed.