The device, placed in a plastic bag just outside a commercial centre in the Zalka area, blew out the walls of the building on Monday, strewing the area with rubble, twisted metal and glass.
"We were sitting in this cafe when suddenly the glass shattered all over us," said Georges Yazbeck, a witness. "We didn't know what had happened."
A series of bombings has hit Lebanon since the February assassination of former prime minister Rafiq al-Hariri, which some Lebanese blamed on Syria.
Damascus denies any role. It was not clear who was targeted by the latest explosion.
Aid workers picked through the rubble of the commercial centre which includes a hotel, cafes and shops. They treated at least four people on site, the Red Cross said.
The device weighed no more than 5kg, a security source said, but it caused panic among residents.
"The sound was terrifying, very strong, I was astonished when I went down and learned that no one was killed, thank God" Ani Nahabidia, a resident in Zalka told Aljazeera.net.
"I was sitting on the balcony with my children when we saw a flash and heard a loud noise," said Fadi Yacoub.
"The kids fell about screaming. We were all covered with dust and broken glass."
Five bombs have killed at least four people and wounded about 50 in Christian areas since the killing of al-Hariri plunged Lebanon into its worst political crisis since the 1975-1990 civil war.
Syria withdrew its forces from Lebanon in late April under pressure from the international community and large Lebanese street protests, sparked by al-Hariri's killing.
A United Nations team investigating the assassination has asked for an extension of its mandate to continue the investigation, UN officials said.