Soon after the blast erupted, crowds began blocking roads, burning tires and shooting in the air in Sunni areas of Beirut and northern Lebanon. In the eastern Bekaa Valley, angry protesters closed the border crossing that links Lebanon with Syria.

In the northern Lebanese city of Tripoli, pro- and anti-Syrian groups fought with rocket-propelled grenades and automatic rifles. One person was killed, according to security officials. Elsewhere, clashes pitted gunmen in a Sunni neighborhood against those in an Alawite neighborhood of the city. Assad belongs to the tiny Alawite sect, an offshoot of Shiite Islam.

Even as the rubble smoldered in Beirut, Lebanon’s fractious political leaders began taking sides. Syria ally Hezbollah condemned the attack and expressed a ‘‘state of great shock over this terrible terrorist crime.’’

But the anti-Syrian blocs placed the blame squarely on Assad.

Former Prime Minister Saad Hariri, a Sunni politician whose powerful father was assassinated in 2005 in a massive truck bombing along the Beirut waterfront, said the Lebanese people must not remain silent about this ‘‘heinous crime.’’

Asked who he blamed, Hariri said: ‘‘Bashar Hafez Assad.’’

Speaking to the Al Arabiya TV station, Walid Jumblatt, leader of the Druse sect, also accused Assad of being behind the killing.

‘‘He is telling us that even though he turned Syria into rubble, ‘I am ready to kill in any place'’’ Jumblatt said.

For much of the past 30 years Lebanese have lived under Syrian military and political domination.

That grip began to slip in 2005, when former Prime Minister Rafik Hariri was assassinated. Syria was widely accused of involvement — something it has always denied — and Damascus was forced to withdraw its troops.

But the killings of anti-Syrian figures continued for several years, and opponents of Assad say he has maintained his influence through proxies in the government.

‘‘The fate of Syria and Lebanon were, are and will always be inextricably linked, for better or worse,’’ said Bilal Saab, a Syria expert at the Monterey Institute of International Studies.

The last major bombing was in 2008, when a car bomb killed a senior Lebanese anti-terror police official who was investigating dozens of other bombings. Four others were killed and 38 wounded in the blast in the Christian Hazmieh neighborhood.

After that, there was relative calm until the uprising against Assad began in March 2011. Since then, there have been sporadic gun battles between pro- and anti-Assad factions, particularly in northern Lebanon.

The explosion shocked a city known for its rollicking nightlife into silence Friday night, as streets that normally would have been packed with cars and partygoers were largely deserted.

‘‘I'm very worried about the country after this explosion,’’ Beirut resident Charbel Khadra said. ‘‘I'm worried the explosions will return — and this is just the first one.’’

‘‘It is a message,’’ said Marwan Rifa, who was working as a legal assistant near the explosion site. ‘‘And the message is, you will never live in peace.’’

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