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."
The White House condemned the attack as a "pattern of political assassinations and attempted assassinations" aimed at intimidating those working for democracy in Lebanon.
One of Ghanem's daughters, Mounia, was at the hospital where her father's body was transported.
She said she had been driving back to the capital from the mountains when she saw plumes of smoke rising from Sin el-Fil.
"Something told me that it was him. I had just spoken to him this morning and told him to be careful," she said.
"He had been outside the country for two months and just came back on Sunday. He didn't have the means to protect himself like the others but he was very fatalistic."
The bomb ripped apart Ghanem's black Chevrolet and killed two of his bodyguards who were with him, his daughter said.
Ghanem's death has reduced the ruling majority in parliament to 68 members out of the 128-member house, with presidential elections due this month.
The killing comes three months after Walid Eido, another March 14 MP, died in a bomb blast in central Beirut.
Wael Abu Faour, a Druze MP, said: "Syria wants to prevent the majority from remaining a majority by assassinating its members.
|A soldier secures the site of the blast [AFP] |
"Syria wants to prevent the majority from electing a president from among its ranks."
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.
Toufic, the owner of a flower shop near the scene, said: "I saw Antoine Ghanem all bloodied, his eyes swollen. He had on a striped blue shirt and a bloodied overcoat.
"May God help us. I can't live in this country any more."