A previously unknown group, "Strugglers for the unity and freedom in the Levant," claimed responsibility in a statement faxed to media outlets in Beirut. It said the group "succeeded in liquidating another bullhorn spreading poisons and lies despite our repeated warnings to him."
The statement's authenticity could not be independently confirmed.
The news triggered an outpouring of grief as Tueni was a respected columnist and the general manager of An-Nahar, the country's leading newspaper.
Bells of Orthodox churches tolled in the Christian quarter of Ashrafieh, Tueni's constituency. Prime Minister Fuad Saniora called an emergency meeting of top security officials and asked the Cabinet to convene later Monday.
An elderly man wept openly at the scene of the bombing, pounding his head and shouting: "My God, Gibran, you were the only one who told the truth!"
At An-Nahar offices, staff with tears in their eyes received diplomats and other dignitaries who arrived to give their condolences. Scores of people, many of them students, gathered outside the paper's offices in downtown Beirut.
Tueni's uncle, Telecommunications Minister Marwan Hamadeh, and the leading Lebanese politician Walid Jumblatt blamed Syria for the bombing — a charge that Syria promptly denied.
Police said Tueni was one of three people killed when a car bomb exploded as his motorcade drove through the hilly industrial suburb of Mkalles. The others were his driver and an unidentified passerby. Another 30 people were wounded in the bombing, which started a fire that destroyed at least 10 vehicles.
Preliminary estimates put the bomb's size at 88 pounds of TNT. The blast tossed several cars, including Tueni's armor-plated vehicle, into a ravine and shattered the windows of nearby shops and buildings.
Lebanese troops and rescuers looked in the bush for body parts. Journalists covering the explosion looked numb as they filed their reports. One reporter broke down in tears when she heard Gibran was dead.
"God have mercy on Gibran and An-Nahar will remain the beacon for freedom," Jumblatt told LBC television.
The political leader of the Druse community, Jumblatt said the bombing was intended to silence a voice who had sought those responsible for the February assassination of former Prime Minister Rafik Hariri.
"This is a new terrorism message," Jumblatt said of the killing, referring to Hariri's assassination and the mysterious series of subsequent bombings that have targeted mainly Lebanese opposed to Syrian influence in Lebanon.
Hamadeh threatened to withdraw from the Cabinet with two colleagues if the government did not demand a U.N. investigation into the continuing series of bombings. He said there must be an international tribunal to "investigate the continuing crimes of the Syrian regime."
Syrian Information Minister Mehdi Dakhlallah denied his government was involved, telling LBC television: "Those who are behind this are the enemies of Lebanon."
Tueni, 48, had only returned to Lebanon on Sunday from Paris, where he has been staying most of the past few months out of fear for his safety.