The deadly attack, allegedly carried out by Daesh, came a day after the regime had declared victory over the militant group in the eastern Syrian city.
At least 75 civilians were killed in a Daesh car bombing that struck a gathering of people displaced by fighting in eastern Syria, a monitor said on Sunday.
Deir Ezzor, on the west bank of the Euphrates River, is the largest and most important city in eastern Syria, and is the centre of the country's oil production.
The Syrian regime on Friday declared victory over Daesh in Deir Ezzor, a big blow to the terrorist group as their last stronghold in Syria crumbles.
Saturday's attack in the eastern province of Deir Ezzor killed "at least 75 displaced civilians including children" and wounded 140, said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights (SOHR).
The war monitor, which relies on a network of sources on the ground inside Syria for its information, reported Saturday that dozens had died in the blast.
SOHR said the attack, carried out by Daesh, targeted people displaced by fighting in the oil-rich province who had gathered on the eastern bank of the Euphrates River.
Abdel Rahman said the victims had fled battlegrounds in Deir Ezzor province.
Sonia Khush, Syria director at the Save the Children charity, said an estimated 350,000 people have fled the recent fighting in Deir Ezzor province, half of them children.
"The situation in the city, and surrounding countryside, has been especially bleak with civilians trapped between the fighting and all too often caught in the crossfire," she said
The Euphrates Valley border area was the heart of the "caliphate" Daesh proclaimed in 2014 and is now its last redoubt, where a US-led coalition supporting the military effort said around 1,500 militants remained.
Despite being driven out from large parts of Deir Ezzor, Daesh still controls 37 percent of the province and its militants are deployed in the eastern side.