"Yet again, we construction workers have to pay with our lives at our work places and nothing is done about it," Johan Lindholm, president of the construction workers' trade union Byggnads, told news agency TT.
In an interview with Swedish Radio News, Lindholm said conditions at the site were like the "Wild West".
The accident happened just before 9 a.m. on Tuesday when a wall collapsed at the site where the thermal power station is being built. Fortum and the City of Stockholm are joint owners of the power station.
According to Fortum, two people were killed on the spot and another person who is in his thirties was seriously injured and taken to hospital. He is fighting for his life, according to Mirjam Kontio, a spokesperson for the Karolinska University Hospital.
A witness told TT that he was about to finish his shift when he suddenly heard a powerful blast. He said a large wall had collapsed over a sky lift where one person was working. The lift fell over two people who were standing on the ground.
Ulf Wikström, Fortums' head of sustainability, said the construction site meets all security requirements even though the company has not agreed to a collective bargaining agreement for its employees, another point of criticism from the union.
"We have fully secured work environment requirements for the project... Collective bargaining agreements do not cover work environment issues, they regulate wages. Work environment issues are governed by Swedish law, which we absolutely follow," Wikström told Swedish Radio News.