At least 239 others were wounded, according to state-run National News Agency.
A would-be suicide bomber who survived the attack told investigators he was an ISIS recruit, a Lebanese security source said. The man, a Lebanese national from Tripoli, Lebanon, was taken into custody after the blasts. He told authorities that he and three other attackers arrived in Lebanon from Syria two days ago, the source said.
Lebanese intelligence believes the bombers could be part of a cell dispatched to Beirut by ISIS leadership, the source said, but investigators are still working to verify the surviving suspect's claim. The three other bombers were killed in the explosions.
In a purported ISIS statement circulated on social media, the terror group claimed responsibility for the blasts. CNN hasn't confirmed the authenticity of the statement.
The explosions detonated within 150 meters (490 feet) and five minutes of each other, Lebanon's state-run National News Agency said, shaking an open-air market and other parts of the Bourj al-Barajneh district in southern Beirut.
"There's a lot of shattered glass on the street, a lot of blood, and it's really just a scene of chaos and carnage," journalist Tamara Qiblawi told CNN shortly after the blasts.
Bombs strike where Hezbollah has strong presence
Three local members of Hezbollah were among those killed in the attack, the Lebanese security source said. The members do not appear to have been a target in the attack and were in "the wrong place at the wrong time," the source said.
The Lebanon-based, Iran-backed Shiite militia has a strong presence in the area where the blasts occurred.
Police are investigating whether two of the bombers were Palestinians from a nearby refugee camp where ISIS has been recruiting, a Lebanese government source said.
In addition to the human toll, the explosions damaged at least four nearby buildings. Video distributed by Reuters showed a dramatic scene in the bombings' aftermath, with rescue workers carrying out victims past piles of rubble and through a mass of people.
And the attackers may have had other targets in mind. One of the suicide bombers tried to get inside a Shiite mosque in the area but was stopped, the government source said.
After the blasts, authorities closed all entrances to Bourj al-Barajneh, NNA reported. Judge Sakr Sakr dispatched military police and other authorities to investigate the blasts, cordoning off the area around them.
Citizens have been urged to stay away from the bloody scene as well as nearby hospitals so that ambulances can more easily get back and forth.
Prime Minister Tammam Salam declared Friday a day of mourning for the victims of the bombings, a terrorist attack condemned by officials across the country's political landscape.
Bombings not new to Lebanon
The country has seen plenty of violence involving numerous parties in recent decades, including the current fallout from the bloody civil war in neighboring Syria.
That war has flooded Lebanon with more than a million refugees, according to the United Nations, and also contributed to intermittent spillover violence.
Most of that bloodshed has been concentrated near the Syrian border, though not all, as evidenced by a November 2013 Beirut bombing
that killed at least 23 people and wounded about 150 more.
The al Qaeda-linked militant group Abdullah Azzam Brigades claimed responsibility for that bombing and warned of more to come unless Hezbollah stopped sending fighters to support Syrian government forces.