Mohamad Chatah, a former Lebanese minister of finance and ambassador to the United States, was killed Friday when a car bomb struck his convoy in downtown Beirut, Lebanon's National News Agency reported.
Four others were killed and 71 were wounded, Lebanon's health ministry said. Cars were burned beyond recognition as a wall of flames and thick black smoke shot up from the blast site.
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 that Chatah was at odds with.
"#Hezbollah is pressing hard to be granted similar powers in security & foreign policy matters that Syria exercised in Lebanon for 15 yrs," Chatah tweeted.
Chatah was known as a staunch critic of Syrian President Bashar al-Assad, whom he accused of meddling in Lebanon's internal affairs. Chatah also opposed Hezbollah, which 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."
Chatah graduated from American University in Beirut and served as Lebanon's ambassador to the United States from 1997 to 1999, according to his blog.
He also served as a senior adviser for former Prime Minister Saad Hariri.
Current 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.