Hariri was travelling in a motorcade when the blast happened
The blast, which reports say killed about nine people and injured 100 others, may have been a car bomb.
It went off beside the derelict St Georges Hotel on the seafront, causing widespread damage.
The killing comes at a time of rising tension between Syria, Lebanon's political master, and members of the opposition, a BBC correspondent says.
Mr Hariri resigned as prime minister and joined the opposition last October. He was hoping to stage a comeback in legislative elections next May.
It is still unclear what caused the massive explosion, but a little-known group calling itself Victory and Jihad in Greater Syria has issued a statement claiming the killing, saying it was a suicide bomb, says the BBC's Kim Ghattas in Beirut.
The authenticity of the statement could not be verified.
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
Lebanon's economic champion
Lebanese opposition leaders have said they hold the Lebanese and Syrian governments responsible for the killing.
"We hold the Lebanese authority and the Syrian authority, being the authority of tutelage in Lebanon, responsible for this crime and other similar crimes," they said in a statement after a meeting held at the late leader's house in Beirut.
They also called for the government's resignation, for Syrian troops to withdraw from the country before the May elections and for a three-day strike.
The White House condemned the attack and said Lebanon should be allowed to pursue its political future "free from violence... and free from Syrian occupation".
Spokesman Scott McClellan said he was not trying to link Syria with the bombing, adding Washington did not know who was responsible, Reuters news agency reported.
Syrian President Bashar al-Assad condemned Monday's attack as a "terrible criminal act".
Lebanon's former colonial power France, whose leader, President Jacques Chirac, had close ties with Mr Hariri, has called for an international inquiry into the blast.
A three-day mourning period has been declared in Lebanon.
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.
The explosion left a huge crater
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, creating a huge crater.
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.
Later, supporters of the late prime minister took to the streets in Beirut and in his hometown of Sidon, in southern Lebanon, people burnt tyres on the streets in protest.
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, our correspondent says.
Mr Hariri had recently joined calls by opposition politicians for a withdrawal of Syrian troops from Lebanon.
France and the US had also been calling on Syria to end its meddling in Lebanese political affairs and to withdraw its troops from its smaller neighbour, our correspondent says.
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.
A large blast on the seafront of west Beirut, Lebanon has killed several people, including former Prime Minister Rafik Hariri.

Current reports conflict on the total number of casualties, suggesting nine or ten people killed and around 100 injured. Hariri was travelling in a car in a motorcade when the explosion occurred around 1pm local time. His bullet-proof car was totally destroyed. The attack took place near the luxurious Saint George hotel, in a residential neighborhood.

The cause of the blast is currently unknown for certain but it is believed to have been a car bomb. Anonymous sources have said car containing 300 kg of TNT was used; however other reports have suggested the explosives were placed in a building. The explosion created a crater at least 10 m across and devastated the fronts of nearby shops and hotels, destroying several buildings entirely. Apartment windows within a 1km radius were blown out.

Only one group has claimed responsibility, a little-known organisation from Syria called Victory and Jihad in Greater Syria. There has been no confirmation that this group actually carried out the attack. Hariri's supporters see Syria as being responsible for the attack. Lately, Hariri had become a chief opponent of the Syrian military presence (14,000 troops) in Lebanon.

The BBC quotes the group has having said "We hold the Lebanese authority and the Syrian authority, being the authority of tutelage in Lebanon, responsible for this crime and other similar crimes."

Rafik Bahaa Edine Hariri (November 1, 1944–February 14, 2005) was a Lebanese self-made billionaire businessman, and was twice Prime Minister of Lebanon, before his last resignation from office on October 20, 2004.