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Hariri was travelling in a motorcade when the blast happened

The blast, which reports say killed about nine people, caused widespread damage and left about 20 cars ablaze.

The bomb went off beside the derelict St Georges Hotel, on the seafront, creating a huge crater.

The assassination comes at a time of rising tension between Syria, Lebanon's political master, and members of the opposition, a BBC correspondent says.

Syrian President Bashar al-Assad condemned Monday's attack as a "terrible criminal act".

Arab League Secretary General Amr Mussa said it was a "terrorist act" and expressed fears about the political fallout, AFP news agency reported.

Beachfront attack

Mr Hariri, who was also an MP, attended a session at parliament in central Beirut shortly before the blast.

He was apparently heading home along the beachfront in a convoy when the explosion happened just before midday local time (1000 GMT), in a busy area full of hotels and banks

RAFIK HARIRI Prime minister 1992-98 and 2000-04 Self-made billionaire Trained as a teacher, but founded a successful construction company in Saudi Arabia Born in 1944 to a poor Sunni Muslim family in the southern Lebanese port of Sidon Obiturary: Rafik Hariri

Members of his convoy are believed to have been killed in the blast. A former minister who was in the convoy is said to have been seriously injured.

The force of the blast left vehicles smouldering and shop fronts blown out and blackened.

Local television pictures showed a burning man fighting to get out of a car through its window, falling to the ground and being helped by a bystander.

Several young women were seen with blood running down their faces.

Lebanese security forces cordoned off the area with yellow tape as rescue workers and investigators combed the scene.

Leading politician

Mr Hariri has been the leading Lebanese politician since the end of the civil war in 1990, and prime minister for most of the last 15 years.

He was also a self-made billionaire businessman.

He resigned in October amid differences with Lebanon's pro-Syrian President, Emile Lahoud.

Since then, he had been considered part of the opposition, although he never formally attended their gatherings, the BBC's Kim Ghattas in Beirut says.

Mr Hariri had recently joined calls by opposition politicians for a withdrawal of Syrian troops from Lebanon.

He was expecting to make a comeback in upcoming legislative elections in May.

Last October, a former minister and member of the opposition was injured in a car bomb attack in Beirut, in which his bodyguard was killed.