A number of sources reported that the motorcade of former Lebanese Prime Minister Rafik Hariri -- who has led the country for most of the period since its bloody 1975-90 civil war ended -- was in the area at the time of the blast.
Lebanon's official news agency reported that Hariri was seriously wounded in the incident, but other reports, quoting state television, said he was killed.
Initial reports suggested the explosion was the result of a car bomb, but that information remains unconfirmed.
Emergency crews were reportedly quick to arrive on the scene, which CNN is describing as one of "widespread devastation."
Tens of cars were reported destroyed, and television images show several buildings that appear to have been heavily damaged and many wounded being treated in hospital or assisted from the scene.
Hariri -- a billionaire who is among the strongest Sunni politicians in the country and has strong U.S., European, and Saudi ties -- remains a prominent force on the Lebanese political scene in opposition since leaving office in October.
Hariri has been engaged in a political rivalry with pro-Syrian President Emile Lahoud for years.
Explosions in Beirut -- common during Lebanon's 15-year civil war -- have become rare since that conflict ended, with a car-bomb attack seriously injuring an opposition politician and killing his driver.
(AP/Reuters/CNN)[For more on the regional implications of events in Lebanon, see "Implications Of New International Consensus On Lebanon" in "RFE/RL Iran Report".]