A double suicide bombing outside the Iranian embassy in Beirut killed at least 23 people on Tuesday, in an attack claimed by an Al-Qaeda-linked group.
The army said a motorcyclist blew himself up moments before a suicide bomber blew up a four-wheel-drive vehicle in the south Beirut stronghold of Hezbollah, an ally of both Iran and the regime in neighbouring Syria.
The mid-morning attack, which the health ministry said also wounded 146 people, was the first time the Iranian embassy had been targeted.
The blasts ripped the facades off surrounding buildings, strewing rubble and glass on streets that were stained with blood.
Residents walked dazed past charred cars and trees, as soldiers and Hezbollah security men tried to secure the area.
The attack followed two other bombings in Hezbollah bastions in Beirut this year, amid rising tensions over the conflict in Syria.
Iran is one of Syria's closest allies, and is the key sponsor of Hezbollah, a powerful Shiite movement that has dispatched thousands of fighters to bolster the regime in its efforts to crush the 32-month uprising.
The blasts were claimed by the Abdullah Azzam Brigades, a jihadist group linked to Al-Qaeda that has previously fired rockets at Israel from Lebanese territory.
"This is a double martyrdom operation carried out by two heroes from the heroic Sunnis of Lebanon," Sirajeddin Zreikat, a member of the group, wrote on Twitter.
Syrian state television said the "government firmly condemns the terrorist attack carried out near the Iranian embassy in Beirut."
It said an "odour of petrodollars comes from all the terrorist acts against Syria, Lebanon and Iraq," an apparent reference to Saudi Arabia and Qatar, which back the rebels in Syria.
Iranian foreign ministry spokeswoman Marzieh Afkham accused Israel and its "mercenaries" of responsibility.
Israel immediately denied involvement.
Speaking in Rome, Iranian Foreign Minister Mohammed Javad Zarif said "the tragedy today... should be an alarm for all of us that we need to deal with and, unless we deal with it seriously, it will engulf all of us."
Asked about Afkham's claim about Israel, he said "we have reason to be suspicious of every move they make."
For his part, Hezbollah MP Mohammad blamed a "Western-Israeli alliance" and regional powers that back hardline Sunni groups, another apparent reference to Saudi Arabia and Qatar.
In a condolence call to his Iranian counterpart Hassan Rouhani, Lebanese President Michel Sleiman said security for Iranian diplomats in beirut should be strengthened, the Iranian president's website said.
"Lebanon's government will do its best to identify the elements behind this terrorist attack and bring them to justice," it quoted Sleiman as saying.
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Britain and France issued swift statements of condemnation, and UN chief Ban Ki-moon urged restraint.
The United States described the attacks as "senseless and despicable", while Russia denounced a "terrorist act".
The army said "the first explosion was caused by a suicide attacker who was riding a motorcycle and blew himself up. The second suicide attacker was driving a 4x4 vehicle and also blew himself up."
Iranian ambassador Ghazanfar Rokn-Abadi said all staff inside the embassy at the time escaped unharmed.
But officials in Tehran said the cultural adviser, Ibrahim al-Ansari, was in critical condition and a medic later said he had died of his injuries.
The twin bombs prompted organisers to order Lebanon and Iran to play a qualifying game for the 2015 Asian Cup in an empty Beirut football stadium on Tuesday. The hosts lost 4-1.
'Act of savagery'
An AFP correspondent at the scene of the blasts described blood and glass on the streets, and media broadcast harrowing images of charred bodies, some still on fire.
One shocked resident said the attack was an act of "savagery."
"People want to live. After this kind of thing, we are paralysed for days. Thank God my children were at school," said Farah, a woman in her 30s.
At the nearby Rasul Aazem hospital, which received seven bodies from the blasts, relatives waited to hear news of their loved ones while others queued to donate blood.
At the Zahraa hospital, Mohamed al-Hajj was searching for his neighbour Tariq.
"He works in front of the embassy and now we don't know where he is. We checked all the hospitals," he told AFP.
Hezbollah has already seen its strongholds in southern Beirut targeted twice by car bombs this year, on July 9 and August 15.
While the first attack caused no fatalities, the second killed 27 people.
Hezbollah's intervention in Syria in support of the Damascus regime has angered many Lebanese Sunnis who back the rebels.
It has also raised fears Lebanon could be engulfed by the Syrian conflict, which has killed more than 120,000 people since March 2011.
But Hezbollah chief Hassan Nasrallah pledged just last week that he would not withdraw his forces.