(CNN) -- Mohamad Chatah, a former Lebanese finance minister and ambassador to the United States, died Friday when a car bomb struck his convoy in downtown Beirut, Lebanon's National News Agency reported.
The blast killed six others and left 71 wounded, Lebanon's state news agency said. Cars were burned beyond recognition as a wall of flames and thick black smoke shot up from the blast site.
Chatah's bodyguard, Mohammed Badr, was among those killed, the news agency said.
No one has claimed responsibility for the attack.
Chatah's last tweet, posted about an hour before his death, talked about Hezbollah, the Lebanese-based Shiite militant group with which Chatah was at odds.
"#Hezbollah is pressing hard to be granted similar powers in security & foreign policy matters that Syria exercised in Lebanon for 15 yrs," Chatah tweeted.
That group decried the attack in a statement aired on Hezbollah TV, saying the it "only benefits the enemies of Lebanon."
The group called on "all the security and judicial agencies to be on high alert to expose the perpetrators and bring them to justice."
Chatah was known as a staunch critic of Syrian President Bashar al-Assad, whom he accused of meddling in Lebanon's internal affairs. Hezbollah has sent fighters to help al-Assad's forces in the Syrian civil war.
"A united and peaceful Syria ruled by Assad is simply not possible anymore. It has been like that for some time," Chatah wrote in his last blog post. "The status quo ante cannot be restored. Iran and Hezbollah realize this more than anyone else."
U.S. Secretary of State John Kerry condemned the killing of Chatah, whom he called "a voice of reason, responsibility and moderation."
"His presence will be missed, but his vision for a united Lebanon, free from sectarian violence and destabilizing interference, will continue to guide our efforts," Kerry said in a written statement. "Indeed, his tragic end reminds all of us just why his vision remains so imperative."
Syrian war spills over into Lebanon
More than 100,000 people have died in Syria's civil war in which al-Assad's forces are battling rebels seeking an end to his family's four-decade dynasty. Al-Assad and the core of his regime are Alawites, members of an offshoot of Shiite Islam, but most Syrians and a large portion of the rebels are Sunni.
The war has spilled across the border into Lebanon. Dozens have been killed there in largely sectarian violence, including dual bombings last month that left 23 dead in Beirut.
The Abdullah Azzam Brigades, a Sunni jihadist group linked to al Qaeda, claimed responsibility for those bombings. The group warned that more attacks would come unless Hezbollah stopped sending fighters to support Syrian regime forces.
Chatah graduated from American University in Beirut and was Lebanon's ambassador to the United States from 1997 to 1999, according to his blog. Chatah also was a senior adviser for former Prime Minister Saad Hariri and was the finance minister in former Prime Minister Fouad Siniora's Cabinet in 2008 and 2009.
Hariri issued a statement Friday condemning the killing, calling it "yet another terrorist message sent to us, we the free men of Lebanon in the Future Movement and March 14 coalition."
Hariri's "March 14 coalition" is a pro-Western, Sunni-dominated bloc. It takes its name from the day in 2005 when thousands gathered in Beirut a month after the assassination of Hariri's father, former Prime Minister Rafik Hariri, to demand an end to the Syrian occupation of Lebanon.
Saad Hariri said the bombers "want to assassinate Lebanon and humiliate and weaken the State."
"The signatories of the message do not hide their fingerprints," he said. "They will continue on the criminal path and will insist on dragging Lebanon into the abyss of discord, as long as there is in Lebanon some who provide cover for those crimes, who ask to bury the heads in the sand and who justify the proliferation of weapons and the rise of the armed groups at the expense of the State and its institutions."
On Friday, Lebanese Prime Minister Najib Mikati posted a tweet saying he is calling off his vacation and heading back to Lebanon.
"I condemn this assassination, which targeted a political, an academic, a moderate and an upscale figure who always believed in dialogue and the language of reason, logic and the right to have a different opinion," Mikati said.
Chatah was married and had two children.
His death comes 20 days before the U.N.-backed Special Tribunal for Lebanon begins the trial of four Hezbollah suspects over the 2005 assassination of Rafik Hariri. Like Chatah, Hariri was also killed in a car bombing.
CNN's Chelsea J. Carter, Jill Dougherty and Nada Husseini contributed to this report.