Three rescuers are among the dead so far, the head of the State Fire and Rescue Service Oscar Abolinsh told local media, saying that the rescue operations will continue until morning.
"We can't conduct the work faster, because we need to first clear the building from large concrete panels and then clear the smaller debris."
Some 500 square meters of roof caved in at the 'Maxima XX' building in the capital, Riga, on Thursday night.
Following the incident, the capital's mayor Nils Ušakovs said that 70 people were trapped under the debris. At the moment, at least 30 are feared to still be in the rubble, the local State Fire and Rescue Service told RIA.
The initial collapse in Riga's densely populated area was followed by a second cave-in just as the first responders at the scene were helping the victims. Three rescuers and firefighters were killed by the second collapse.
So far rescuers have managed to save some 40 people from the rubble. At least one child has suffered a moderate head injury and has been hospitalized along with dozens of victims, RIA Novosti reports. Latvia will be in official mourning from November 23-25.
While the cause is currently unknown, Riga Vice Mayor Andry Ameriks refuted earlier eyewitness reports of an explosion and attributed the incident to a likely construction fault.
"Probably, mistakes were made by construction workers, which led to the collapse of the building," Ameriks said. "The building collapsed completely. Now, all the rescue services are working at the scene. The most important thing now are the lives of the people."
A total of 400 people are working at the scene including local and state police, some local home guards and at least 17 units of medics as well as 13 fire brigades. Soldiers of the National Armed Force are also helping look for the victims with dog units. Another 40 soldiers were sent from the army garrison, local media reports.
Council official Juris Radzevics confirmed that the roof of the supermarket, built in 2011, was in the process of being turned into a green area.
"The project was submitted in accordance with all regulations but of course we will be looking at whether materials and work were [of] the proper standards," Mr Radzevics told Latvian television.
Police are looking into several possible reasons for one of the worst building collapses in the Latvian capital:
Constructions work on the roof, as witnesses told the media, a winter garden was been built there; secondly, faults in the building's original design; and builders using the roof to keep building materials, which then collapsed because of the excess weight.
