Lebanon's cabinet met in emergency session on Saturday as scattered protests erupted around the country, a day after a top security official was killed in a car bombing blamed on Syria.

Saturday was declared a day of mourning and Internal Security Forces (ISF) intelligence chief General Wissam al-Hassan was to be buried on Sunday alongside former premier Rafiq Hariri, murdered in a car bombing seven years ago.

The anti-Syrian opposition, led by Hariri's son Saad, called for a massive turnout for the funeral, which will be held after Muslim prayers at the Al-Amine Mosque in the capital's centre.

Hassan, who investigated the murder of the elder Hariri in a still-unsolved crime widely blamed on Syria, was among at least three people killed in Friday's powerful rush-hour explosion in Ashrafieh, an upmarket mostly Christian district of Beirut.

The bombing drew condemnation from abroad, with US Secretary of State Hillary Clinton calling it a "dangerous sign that there are those who continue to seek to undermine Lebanon's stability."

Protesters blocked some roads in Beirut, Sidon in the south, Tripoli in the north and the Bekaa Valley in the east. In some cases, they burned tyres.

On Friday, official figures put the death toll at eight, but the ISF and the Red Cross both lowered the figure on Saturday.

Red Cross spokesman Ayad Mounzer told AFP "this is not the final toll, as we still need to see what happens to the critically wounded."

The ISF put the number hurt at 80 in a blast that ripped through a busy square in the district where Hassan lived, and the Red Cross at 110.

The site, a mass of rubble and twisted metal, remained blocked off and investigators were sifting through the damage.

The cabinet had gone into emergency session mid-morning to discuss the fallout from the bombing, after key opposition groups called on the government to step down.

"The government must leave and we call on Prime Minister Najib Mikati to resign immediately," Ahmad Hariri, secretary general of the anti-Assad Future movement, said on Friday night.

Hassan, 47, was close to Saad Hariri, himself a former premier who leads the major opposition March 14 coalition of which Future is a party.

In keeping with a complex power-sharing arrangement in multi-confessional Lebanon, the premier is always a Sunni Muslim. But parliament and the government are dominated by the powerful Shiite movement Hezbollah, an Assad ally.

Mikati's office said the "size and tragic consequences of this heinous crime is a source of severe pain and sadness to the prime minister."

Soon after the bombing, Syria condemned what it called a "terrorist, cowardly" attack.

But both Saad Hariri and Walid Jumblatt, the influential Druze leader, accused the Syrian president of being behind the bombing.

"We accuse Bashar al-Assad of the assassination of Wissam al-Hassan, the guarantor of the security of the Lebanese," said Hariri.

Jumblatt told AFP "the Syrian regime is expert in political assassinations. Our response needs to be political. A president who burns Syria and is the executioner of Damascus does not care if Lebanon burns."

Hezbollah said the attack was "an attempt to destabilise Lebanon and national unity."

Following the news that Hassan, a Sunni, had been killed, angry Sunnis set fire to tyres and blocked the road from the northern coastal city of Tripoli to the Syrian border.

Also in Tripoli, gun battles broke out after the office of pro-Hezbollah Sunni party Tawhid came under rocket fire. A Sunni sheikh and party member, Abed al-Razzak al-Asmar, was killed in the crossfire, a security official said.

Traffic was light on the streets of Beirut on Saturday morning, while news of the explosion and its implications for the country's fragile political structure dominated the press.

"Wissam al-Hassan martyred... and civil peace in danger," said a headline in the As-Safir newspaper.

"Tomorrow will not be like yesterday. The assassination of Wissam Hassan will not pass. Lebanon will shift from the era of waiting for the worst, to living under the worst and with the costliest risks".

An-Nahar wrote that "the iron spear against the Assad regime has been assassinated", referring to Hassan.

Under Hassan, the ISF played a central role in the August arrest of former information minister Michel Samaha, closely linked to Damascus, who was charged with planning attacks to instigate sectarian strife.

Friday's bombing touched off painful memories of Lebanon's 1975-1990 civil war and the unrest that has followed, much of it linked to Syrian influence.

At the time of Hariri's murder in February 2005, Lebanon was occupied by Syrian troops who had entered during the civil war under an Arab League mandate.

The outcry that followed Hariri's murder prompted Damascus to withdraw, but Syria still exerts a strong influence.

No one has ever been tried for Hariri's murder, but a UN-backed tribunal has indicted four Hezbollah members who are still at large.