At least 20 people were injured when two cranes and a section of a bridge collapsed in the Dutch city of Alphen aan den Rijn on Monday.
The city lies about 20 miles outside of Amsterdam. Rescue workers are attempting to determine the exact number of casualties.
Dutch public broadcaster NOS reports that many of the victims are seriously injured.
The machines were being used to restore a bridge on the river Rijn when they tipped over, crushing apartments and small shops beneath them, reports The Independent.
Some three hours after the incident on Monday afternoon, the number of people injured remained unclear, and local officials urged the public to stay away from the scene to enable rescue teams to carry out their work into nightfall.
Local fire brigade spokesman Ton Koot said that one person had been taken to hospital. He added that initial reports suggesting that between seven at 20 people were injured had been based on estimates on the number of people who may have been in the buildings.
A video shows the cranes moving a piece of decking on the river Rijn, before they crash onto nearby buildings. Dick van Smirren, who filmed the incident, said: "You see the cloud of dust rise - that is a terrible image and you think, 'Who is under there?' You can't believe what you see."
The Dutch Safety Board, which investigates serious accidents, said it was sending a team to the scene.
Images from the scene show an enormous metal pillar after it smashed into a building, and people standing behind tape cordoning off the area.