"It was a big explosion. We have not seen any explosion in Lebanon since June 14, when a member of parliament was assassinated in Beirut."
She said Ghanem was believed to have been travelling to a meeting in the district.
The Phalange party is part of the larger March 14 Forces bloc, which holds a majority in parliament.
Members of the March 14 bloc have accused Syria of being behind a string of political assassinations and attacks since October 2004.
Speaking from the scene, Robert Fisk, of the UK newspaper the Independent, told Al Jazeera: "Standing where I am you can't say who did it. But this is a very professional killing. This was a very professional assassination... these people knew what they were doing."
Ghanem's killing comes three months after Walid Eido, another March 14 MP, died in a bomb blast in central Beirut.
Nayla Moawad, the Lebanese minister of social affairs, told Al Jazeera of the attack: "It is clearly an attempt to diminish the number of members of parliament from the majority and of the March 14 Forces.
"It proves that there is a mainstream, led by Iran and Syria, against Lebanese sovereignty.
"It is the end of any dialogue. We will not dialogue with murderers and we will not dialogue on the blood of our martyrs."
The bomb attack comes amid continued differences between the parliamentary majority ahead of presidential elections, which are due to begin on September 25.
Television footage showed several cars on fire and bodies being carried away from the scene.
Khodr said several residential buildings had been badly damaged in the explosion, with shattered glass lying on the road.