Twin blasts claimed by the Islamic State group have killed 41 people on a busy shopping street in southern Beirut.
The area is a stronghold of the Shiite movement Hezbollah, and is the worst such attack in years.
More than 200 people were wounded, many of them seriously, said Health Minister Wael Abou Faour at the scene of the explosions, in a narrow shopping street in the Burj al-Barajneh neighbourhood.
The attacks appeared to mark a return to the campaign against Hezbollah strongholds between 2013 and 2014, ostensibly in revenge for the militants' support of regime forces in neighbouring Syria's civil war.
Two men wearing suicide vests carried out the attack, said the army, while the body of a third who had failed to detonate his explosive device was found at the scene of the second blast.
The street in the poor mainly Shiite Muslim neighbourhood, normally home to a market, was stained red with blood according to an AFP photographer, who saw bodies inside nearby shops.
Surrounding buildings were badly damaged by the blasts and security forces were trying to cordon off the scene and keep people from gathering.
Sunni jihadist group IS claimed the attack, saying its "soldiers of the Caliphate" detonated explosives planted on a motorbike on the street, in an online statement.
"After the apostates gathered in the area, one of the knights of martyrdom detonated his explosive belt in the midst of them," the statement added, without referring to Hezbollah's involvement in Syria, much of which is under IS control.
The statement could not be independently verified, but it followed the usual format of IS claims of responsibility and was circulated on jihadist online accounts.
Lebanese Prime Minister Tammam Salam announced a national day of mourning for Friday, local media reported.
The White House offered its condolences for what it described as the "horrific terrorist attacks", vowing that "such acts of terror only reinforce our commitment to support the institutions of the Lebanese state".
Un Secretary General Ban Ki-moon called on Lebanon to "not allow this despicable act to destroy the relative calm that has prevailed in the country over the past year".
The attacks were the deadliest to hit a Hezbollah stronghold since the group entered Syria's civil war in support of President Bashar al-Assad.