About 50 rescue workers and volunteers, including a specialist team from Chile, had yet to locate anyone after sensors on Thursday (September 3) detected signs of breathing and heat.
But they said they would continue while there was a small chance of finding a survivor, and had narrowed their search.
Rescue efforts dominated local and social media, as the Lebanese were transfixed, desperate for some good news.
The ruined building where the search was continuing lies between the residential districts of Gemmayze and Mar Mikhael, among the hardest hit areas by the blast and home to many old buildings that crumbled as the shockwave ripped through.
Work was slow, rescue workers said, as the badly damaged building was at risk of collapse.
Workers were using shovels and their hands to dig, while mechanical diggers and a crane lifted heavy debris.