"There are four victims... Rescue efforts will continue through the night but it's too early to name a cause or say whether there will be more victims or survivors," emergency services spokeswoman Viktorija Sembele told AFP yesterday.
A firefighter working at the scene was among the dead, she said, but she was not able to immediately provide information about the age or sex of other victims.
Riga's deputy mayor Andris Ameriks told the local BNS news agency the disaster could have been caused by a blast, but did not elaborate.
Mayor Nils Usakovs announced he was cancelling his leave and calling an emergency meeting of municipal and rescue services today.
"As soon as rescue work has finished, construction experts will investigate the causes," said a press release from his office.
Up to 13 emergency brigades and 17 ambulance teams were working at the site late yesterday.