By Samia Nakhoul and Stephen Kalin
BEIRUT (Reuters) - Former Lebanese minister Mohamad Chatah, who opposed Syrian President Bashar al-Assad, was killed in a massive bomb blast on Friday which one of his political allies blamed on the Shi'ite Hezbollah militia.
The attack in Beirut killed five other people and threw Lebanon, which has been drawn into neighboring Syria's conflict, into further turmoil after a series of sectarian bombings aimed at Shi'ite and Sunni Muslims over the past year.
Former prime minister Saad al-Hariri accused Hezbollah of involvement in the killing of Chatah, his 62-year-old political adviser, saying it was "a new message of terrorism".
"As far as we are concerned the suspects ... are those who are fleeing international justice and refusing to represent themselves before the international tribunal," Hariri said , referring to the upcoming trial in The Hague of five Hezbollah members suspected of killing his father Rafik in 2005.
U.S. Secretary of State John Kerry said: "This is a terrible loss for Lebanon, the Lebanese people and for the United States."
United Nations Secretary-General Ban Ki-moon condemned the bomb attack, and the 15-member Security Council expressed its "unequivocal condemnation of any attempt to destabilize Lebanon through political assassinations".
The explosion destroyed Chatah's car, turning it into a heap of twisted metal, and injured another 71 people. It took place not far from where the elder Hariri, a former prime minister and influential Sunni figure, was assassinated by a huge bomb in February 2005.
The long-delayed trial of five Hezbollah suspects indicted for that attack, which also killed 21 other people, is due to open in The Hague in three weeks' time.
The suspects are all fugitives and Hezbollah, which denies any role in the Hariri assassination, has refused to cooperate with the court, which it says is politically motivated. Preliminary U.N. investigations implicated Syrian officials.
Hezbollah, which has sent fighters to Syria to help Assad's forces against rebels in the civil war, condemned Friday's blast as a "horrible crime".
The attack on Chatah is linked to a power struggle that has raged in Lebanon since Hariri's assassination. This was followed by a series of attacks which killed anti-Syrian politicians, officials and journalists. In October last year, General Wissam al-Hassan, an intelligence chief linked to Hariri, was killed by a car bomb in Beirut.
Chatah, a vocal critic of Hezbollah, was regarded as one of the brains behind Saad al-Hariri's Future movement and the March 14 opposition coalition, and their point person with Western governments.
A message on Chatah's Twitter account less than an hour before the blast accused Hezbollah of trying to take control of the country, which was occupied by Syrian forces until after Hariri's assassination.
"Hezbollah is pressing hard to be granted similar powers in security and foreign policy matters that Syria exercised in Lebanon for 15 years," the tweet read.
The conflict in Syria has polarized Lebanon and increased sectarian tensions. Hezbollah members are fighting for Assad - who is from the Alawite sect, an offshoot of Shi'ite Islam - while some of the Sunni Syrian rebel groups are linked to al Qaeda, which is also seeking to topple Assad.
Former minister Marwan Hamadeh, who survived a car bomb in 2004, told Al Arabiya television: "Hezbollah will not be able to rule Lebanon, no matter how much destruction it causes or blood it spills."
Hezbollah condemned the blast as "part of a series of crimes and explosions aimed at destabilizing the country and its unity", in a statement issued before Hariri had accused it of involvement.
Iran, which backs Hezbollah, came under attack in Beirut last month. On November 19, two suicide bombings rocked its embassy compound, killing at least 25 people including an Iranian cultural attaché.
Lebanese President Michel Suleiman, Prime Minister Najib Mikati and officials from across Lebanon's sectarian political divide condemned Chatah's killing.
Mikati said the blast targeted "a moderate academic and noble political figure who believed in dialogue, the language of reason and the right to different views".
Fouad Siniora, a leading member of the March 14 opposition movement to which Chatah belonged, said the latest killing would be referred to the Hague-based Special Tribunal.
"The killer is the same one that has been targeting the heroes of Lebanon. The crime occurred... weeks before the opening of the Tribunal in the Hariri assassination," he said.
While Chatah had no political power base of his own, his international experience, diplomatic contacts and academic analysis made him a leading member of Hariri's circle of advisers.
An economist and a diplomat, he worked for the International Monetary Fund in Washington and served as Lebanon's ambassador to the United States. He was also minister of finance from July 2008 to November 2009, after which he worked as a foreign policy adviser to the younger Hariri.
Friday's blast was heard across the city at around 9:40 a.m. (0740 GMT) and black smoke was seen rising in the chic downtown business and hotel district where people seeking a respite from recent turmoil had ventured out to enjoy the Christmas and New Year holidays.
It shattered glass in nearby apartment blocks and damaged cars, restaurants, coffee shops and offices.
"For a while I was thinking, 'Am I still alive?' I didn't know what happened. I was just seeing the people running and holding their ears and eyes, and running," said Maya, manager of the Taten dress shop.
"There was a big ball of fire and panic everywhere and then we learned that Chatah was the target," said another witness, Adel-Raouf Kneio.
Sources at the explosion site said Chatah was on his way to attend a meeting at Hariri's headquarters when the explosion tore through his car. Hariri himself has stayed away from Lebanon for more than two years, fearing for his safety.
Investigators said the blast was caused by a 60 kg (130 lb) bomb. A Reuters witness said Chatah's car was "totally destroyed, it is a wreck". Chatah's identity card, torn and charred, was found at the scene.
Much of Beirut went into lockdown following the explosion, with police blocking off roads across the city. After a series of blasts in the capital and in the northern city of Tripoli, the Lebanese army had stepped up security measures before Christmas and New Year, fearing further attacks.
Workers at dress shops, whose entire glass facade was destroyed, were sweeping up glass, picking up damaged mannequins and counting the damage to the luxury garments.
Surveying smashed windows, a restaurant owner said: "The damage to the glass is not the problem. People won't want to come here now. We were fully booked for the next five days."
(Additional reporting by Mariam Karouny, Leila Bassam and Dominic Evans; Editing by Mark Trevelyan)