Friday's powerful bomb in a residential neighbourhood of the Lebanese capital killed the country's intelligence chief, Brig. Gen. Wissam al-Hassan. Seven other people died and 80 were wounded in the blast in the city's mainly Christian Achrafieh neighbourhood.
Prime Minister Najib Mikati on Saturday said the blast is linked to the intelligence chief's recent case, in which he exposed an alleged plot by Syria to sow chaos in Lebanon.
The UN Secretary-General spoke by telephone on Saturday with Mikati and according to a statement, "offered his deepest condolences for the loss of life, and noted he had issued a strong statement on the attack."
Ban Ki-moon also expressed solidarity with the people of Lebanon and the government as well.
Angry youths burned tires and set up roadblocks around Lebanon on Saturday to protest against the attack and the death of al-Hassan, who led an investigation that implicated Syria and its Lebanese ally Hezbollah in the assassination of former prime minister Rafik al-Hariri in 2005.
The Lebanese cabinet held an emergency meeting Saturday as the country's opposition called for Mikati to resign. The state-run National News Agency said security commanders would attend the meeting to discuss how to keep the peace.
Reuters reported that Mikati offered his resignation but that President Michel Suleiman asked him to stay on for a "period of time".
Many observers said the attack appeared to have links to the Syrian civil war, which has been raging for 19 months. Al-Hassan, 47, headed an investigation over the summer that led to the arrest of former Information Minister Michel Samaha, one of Syrian President Bashar Assad's most loyal allies in Lebanon.
Samaha, who is in custody, is accused of plotting a campaign of bombings and assassinations to spread sectarian violence in Lebanon at Syria's behest. Also indicted in the August sweep was Syrian Brig. Gen. Ali Mamlouk, one of Assad's highest aides.
Al-Hassan also played a role in the investigation of the 2005 truck bombing assassination of Hariri. A UN-backed tribunal has indicted four members of militant group Hezbollah, which along with its allies, now holds a majority in Lebanon's cabinet. Hezbollah denies involvement in Hariri's killing and has refused to extradite the suspects.
Al-Hassan's department also had a role in breaking up several Israeli spy rings inside Lebanon over the past few years, Lebanese officials said.
Syria crisis intensified Lebanese tensions
Lebanon's fractious politics are closely entwined with Syria's. The countries share a web of political and sectarian ties and rivalries, often causing events on one side of the border to echo on the other. Lebanon's opposition is an anti-Syrian bloc, while the prime minister and much of the government are pro-Syrian.
The civil war in Syria has laid bare Lebanon's sectarian tensions as well.
Many of Lebanon's Sunni Muslims have backed Syria's mainly Sunni rebels, while Shia Muslims have tended to back Assad. Al-Hassan was a Sunni whose stances were widely seen to oppose Syria and Shia Hezbollah, the country's most powerful ally in Lebanon.
The government has declared Saturday a day of national mourning, a move seen as a subtle attempt to keep people oftf the streets. A number of main roads in the capital and other cities have been closed ahead of Sunday's funeral for al-Hassan.
Lebanon's top Sunni cleric, Grand Mufti Mohammed Rashid Kabbani, condemned the assassination, calling it a "criminal explosion that targets Lebanon and its people." He called for self-restraint saying that "the criminal will get his punishment sooner or later."
Police and army troops sealed off the site of Friday's blast as military intelligence agents investigated what was the deadliest bombing in Beirut in four years.
On Friday, protesters in mostly Sunni areas closed roads with burning tires and rocks in Beirut, the southern city of Sidon, the northern city of Tripoli and several towns in the eastern Bekaa Valley.
The highway linking central Beirut with the city's international airport was closed, as well as the highway that links the capital with Syria, the officials said on condition of anonymity in line with regulations.