Former Lebanese minister and opposition figure Mohamad Chatah has been killed by a car bomb in central Beirut.
Four others were killed and at least 50 people were hurt in the attack.
Mr Chatah, a Sunni Muslim, was an adviser to ex-PM Saad Hariri. He was also a staunch critic of Syrian President Bashar al-Assad and Lebanon's Shia Hezbollah movement that backs him.
There has been a wave of attacks in Lebanon, linked to heightened Sunni-Shia tensions over the Syrian war.
No-one has claimed responsibility for Friday's attack.
Saad Hariri implicitly accused Hezbollah of carrying out the bombing.
He blamed "those who are hiding from international justice and who have spread the regional fire to the [Lebanese] nation".
For its part, Hezbollah called the bombing a "heinous crime, which comes in the context of a series of crimes and explosions aimed at sabotaging the country".
"No-one benefits from [the bombing] but Lebanon's enemies," a statement from the movement said.
Five Hezbollah suspects are due to go on trial in three weeks' time, charged in connection with assassinating Saad Hariri's father and former Prime Minister, Rafik, in a huge car bombing in February 2005.
Hezbollah has denied involvement in Rafik Hariri's death.
Syria denied any role in Friday's explosion.
Syrian Information Minister Omran Zoab called accusations that Syria was behind the blast "random and arbitrary".
US Secretary of State John Kerry called the attack "abhorrent" and said it underlined the importance of the tribunal in The Hague looking into Rafik Hariri's assassination.
UN Secretary General Ban Ki-Moon also condemned the attack, saying "the recurring acts of terrorism in Lebanon... pose a severe threat to the country's stability and national cohesion".
'Terror and panic'
Mr Chatah was on his way to a meeting of the anti-Syrian March 14 bloc, led by Saad Hariri, when his convoy was hit.
The bomb went off at 09:00 (07:00 GMT) between the Starco Centre and Phoenicia Hotel, not far from the Lebanese parliament building.
The blast damaged several buildings and set several cars ablaze.
Witnesses described shock and fear at the scene of the blast.
"We were opening our store when we heard the blast. It was really loud. We are used to blasts in Lebanon but not in this area. Now we are not safe anywhere," said Mohammad, a shop assistant quoted by AFP news agency.
Adel-Raouf Kneio, who saw the blast, told Reuters news agency the explosion had "caught motorists driving in the morning rush hour" and "there was terror and panic among residents".
"There was a big ball of fire and panic everywhere and then we learned that Chatah was the target," he said.
Forensic experts are at the scene, which has been sealed off by security forces.
'Sending a message'
The BBC's Carine Torbey in Beirut says Chatah was not a controversial figure in Lebanon. He was known as a moderate and so there is a lot of speculation that the bombing was a message sent to the March 14 bloc itself, rather than Chatah as an individual, our correspondent says.
In a Twitter message early on Friday, shortly before he was killed, Mr Chatah said Hezbollah was "pressing hard to be granted similar powers in security and foreign policy matters that Syria exercised in Lebanon for 15 years".
Syria withdrew its forces from Lebanon amid a backlash over the killing of Rafik Hariri, in which many suspected it had a role.
Hezbollah has sent fighters to help President Assad in the war against Sunni-led rebels in Syria. President Assad comes from the Alawite sect, a heterodox offshoot of Shia Islam.
Iran, which backs Hezbollah, saw its embassy in Beirut attacked last month. A Sunni jihadist group Abdullah Azzam Brigades said it had carried out that attack.
Earlier this month, a senior Hezbollah commander with close links to Iran, Hassan Lakkis, was shot dead outside his home near Beirut. A little known Sunni militant group claimed responsibility.