At least nine other people were killed and 100 wounded, including a former economy minister, in the blast, which raised immediate fears that Lebanon — largely peaceful since the 1990 end of its civil war — was headed toward a new and bloody twist in the continuing and divisive dispute over Syria's role.
The Lebanese National News Agency, quoting a statement from American University Hospital, said Hariri was pronounced dead on arrival, his body mutilated in the massive explosion.
Former Economy Minister Bassel Fleihan, a member of parliament in Hariri's bloc, was severely wounded and admitted to the intensive care unit of the American University Hospital, said another pro-Hariri legislator, Atef Majdalani. Hariri's own Future TV reported that Fleihan was in critical condition and the hospital was preparing to transfer him abroad.
An emergency Cabinet meeting was called and Lebanon's supreme defense council — security Cabinet ministers, top leaders and military officials — were in session at the presidential palace, a presidential spokesman said.
Syrian President Bashar Assad said he "condemned this horrible criminal action," according to SANA, Syria's official news agency. Assad urged the Lebanese people to reject those who "(plant) schism among the people" during this "critical situation."
Hariri's assassination removes a main political buffer in a country divided among an opposition strongly opposed to Syria's role, and the pro-Syrian government camp.
Hariri's supporters quickly took to the streets, chanting his praises outside the American University Hospital where he was declared dead. In his hometown of Sidon, supporters blocked roads and burned tires.
The explosion at 12:55 p.m. was so powerful that Hariri's motorcade of bullet-proof vehicles was left a burning wreck.
It was not immediately known whether the explosives had been planted in a car or a building, but they blew a 10-meter-wide crater in the street and shattered windows of hotels and apartment buildings.
At least 20 cars were set on fire in a blast that devastated the front of the famous St. George Hotel, blowing off balconies, and damaged a British bank and the Phoenicia Hotel.
Bystanders and ambulance workers made crude stretchers to carry the wounded to vehicles to take them to nearby hospitals. TV footage showed several men dragging a slain victim partially covered by a brown blanket through the rubble-strewn street before letting go of his arms and letting him fall to the ground. Flames still licked from his body and his face appeared grossly disfigured by burns.
There was no credible claim of responsibility. The Al-Jazeera satellite TV channel said its Beirut office received a call from somebody who said a previously unknown group had killed Hariri.
"This appears to be a very powerful car bomb that affected at least two city blocks in their entirety. Glass was broken in windows from skyscrapers about a mile away from the explosion," CBS Newsman Edward Yeranian reports.
"Explosions have been fairly rare in Beirut over the last several years," said Yeranian. "There was a Lebanese politician that was targeted about 3 months ago, but there were very few victims, and it was a very limited area that was affected."
Hariri was a self-made billionaire who led Lebanon for 10 of the years since the end of the 1975-1990 civil war. He was elected prime minister in 1992 and served until 1998, forming at least three Cabinets. He was elected again in 2000 and served until he resigned in October.
Hariri moved toward the opposition camp after leaving office — in large part because of a dispute concerning Syria's controversial role in Lebanon. Hariri had rejected a Syrian-backed insistence that his old rival, President Emile Lahoud, remain in office as president for three more years.
Pro-Syrian allies of Lahoud accused Hariri of being behind the U.N. Security Council resolution in September that demanded Syria withdraw its army from Lebanon and stop interfering in the country. The resolution was sponsored by the United States and France.
Hariri was credited with rebuilding Lebanon from the destruction of the civil war, but he was faulted with shackling Lebanon with a debt of more than US$35 billion. His wide international business and political connections helped earn Lebanon wide recognition and attracted badly needed foreign investment.
TV footage showed dramatic scenes of one burning man struggling to get out of a car window, then falling on the ground. He was helped by a bystander who used his jacket to put out the flames, but it was not clear if he survived.
Several young women were seen with blood running down their faces. Some had to be helped from the scene.
Heavily armed security forces cordoned off the area with yellow tape as rescue workers and investigators combed the scene apparently looking for casualties or clues to what caused the huge explosion.