Syria has been blamed for a car bombing in Beirut that killed a prominent Lebanese security official and seven others.
Saturday's blast, the worst the Lebanese capital has seen in more than four years, wounded 86 people and was seen as a sign that Syria's civil war is spilling over the country's borders..
When asked who he thought was responsible for the killing, Saad al-Hariri, leader of the opposition March 14 alliance, replied: "Bashar Hafez al-Assad," referring to the Syrian president.
"Who killed Wissam al-Hassan is as clear as day. Certainly the Lebanese people will not be silent over this heinous crime and I, Saad Hariri, promise that I will not be silent."
Walid Jumblatt, Lebanon's Druze leader, also accused Assad of being behind the attack.
"The Syrian regime is expert in political assassinations," he told the AFP news agency. "Our response needs to be political. A president who burns Syria and is the executioner of Damascus does not care if Lebanon burns."
The attack in the Ashrafiyeh neighbourhood killed Wissam al-Hassan, intelligence chief of the Internal Security Forces (ISF), who led an investigation that implicated Syria and Hezbollah in the assassination of former prime minister Rafik al-Hariri, Saad al-Hariri's father.
He also headed an investigation over the summer that led to the arrest of former Information Minister Michel Samaha, an ally of Assad, accused of trying to help smuggle explosives into Lebanon.
Syrian Information Minister Omran al-Zohbi condemned what he called a "terrorist, cowardly" attack. Such incidents were "unjustifiable wherever they occur," he said.
Al Jazeera's Rula Amin, reporting from Beirut, said the blast scene was still cordoned off on Saturday morning and security was tight.
"We can see investigators checking out the scene, even taking stuff in boxes, looking for the smallest evidence to come to a conclusion who was behind this killing," she said.
Protesters angered by the killing have blocked several roads in Beirut and in the southern city of Sidon with burning tyres.
Lebanese ministers were holding an emergency cabinet meeting on Saturday. The government, which is dominated by the powerful Shia movement Hezbollah, has also declared Saturday a day of mourning.
Hezbollah is a strong ally of the Assad regime whose forces are battling a bloody uprising that erupted in March 2011.
Lebanon's opposition called on the government to resign over the attack.
"The government must leave and we call on Prime Minister Najib Mikati to resign immediately," said Ahmad al-Hariri, secretary-general of the Future movement.
Mikati's office said on Saturday that the "size and tragic consequences of this heinous crime is a source of severe pain and sadness to the prime minister", and would be the focus of the cabinet meeting.
Friday's rush-hour car bombing has raised fears of renewed sectarian violence in a country still scarred from a long civil war.
Clashes erupted in the northern city of Tripoli on Friday night between the rival districts of Jabal Mohsen and Bab al-Tabbaneh, leaving one person dead.
Residents of the two neighbourhoods have clashed on several occasions since the start of the Syrian uprising. Jabal Mohsen is resided by mostly Alawites supportive of Assad, while Bab al-Tabbaneh is a Sunni area sympathising with the opposition.
Al Jazeera's Hoda Abdel-Hamid, reporting from Tripoli, said there had been sporadic fighting overnight and that gunfire had been heard in the city on Saturday morning after armed groups urged shopkeepers to keep stores shut.
"People on the street tell us that armed men on motorcycles were shooting in the air, warning shopowners that they must stay shut today," she said.
The main highway from Tripoli to the Syrian border was shut down, with protesters burning tyres in the Akkar area.
Meanwhile, the 15-member UN Security Council called the attack a "heinous act" and appealed to the Lebanese people in a statement to "preserve national unity in the face of such attempts to undermine the country's stability".
The US described the bombing as a "terrorist attack".
Iran also also commented on the bombing, blaming it on its arch foe Israel.
Tehran condemned "the terrorist blast which was carried out by those who aim to create division between different Lebanese groups that never serves the interests of Lebanon," spokesman Ramin Mehmanparast said in a statement on the foreign ministry website.
"Undoubtedly, the main enemy of the Lebanese people and the region is the Zionist regime which benefits from instability and lack of security in the region."
Ziad Baroud, a former Lebanese interior minister, told Al Jazeera that it was too soon to tell who was behind the bombing.
"We have no indication whatsoever [of who is behind this]. We know this is a strong and sad message, and we know this could destabilise the whole country," he said.
The most prominent car bombing since Lebanon's 1975-1990 civil war took place on February 14, 2005, when a massive blast killed Rafik al-Hariri and 22 other people as his motorcade drove along the Beirut waterfront.