Security Council to condemn the attack, perhaps Thursday, said Mladenov, who traveled to the blast scene.
Angry crowds are reported to have gathered in the area immediately following the blast, pelting police and initially refusing to let them and rescue workers reach the scene of the blast.
Mayor Sly James was at the scene after the blast, praising the work of first responders.
President Enrique Pena Nieto and Mexico City Mayor Miguel Angel Mancera headed to the scene of the blast.
Police are examining the scene of the blast in Seacourt Road.
Two police officers who delivered first aid to injured casualties at the scene of a blast at a fireworks depot have been recognised for their bravery.
President Enrique Pena Nieto and Mexico City Mayor Miguel Angel Mancera, who visited the scene of the blast, said Pemex rescue and security teams were working alongside city authorities to help the injured.
BBC: Mexico City blast: Hunt for survivors in Pemex building
Many of the casualties were caused by the second blast as police and media rushed to the scene.
One resident, Jan Ali, told Associated Press that Thursday's blast at the snooker hall was "a scene like hell on Earth".
Issam Ahmed, a journalist with the Dawn newspaper in Lahore who arrived at the scene about 20 minutes after the blast, told the BBC he could still hear shooting in the area.
Sadr City, the scene of Tuesday's deadly blast, is a stronghold of Shiite cleric Muqtada al-Sadr and where some of the capital's most intense fighting in recent months has occurred between Shiite militia members and Iraqi security forces.
The blast killed mostly daily wage workers, police constables at the scene told The Wall Street Journal.
The video and photos provided by federal prosecutors showed the two brothers carrying backpacks as they walk onto Boylston Street, the scene of the bombings, about 11 minutes before the first blast occurred at 2:49 p.m.
WSJ: Dzhokhar Tsarnaev Charged by U.S. Prosecutors in Boston Marathon Attack
At a U.N. compound next door to the scene of the attack, anxious U.N. employees hunkered down indoors after shrapnel from the initial blast sprinkled the gardens.
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