Two collapsed cranes are seen in Alphen aan de Rijn, the Netherlands, Aug 3, 2015. [Photo/Agencies]

AMSTERDAM - Two cranes hoisting a massive section of bridge collapsed in a western Dutch town on Monday, flattening a row of houses and injuring at least 20 people, authorities said.

Dramatic videos of the accident in Alphen aan den Rijn showed the section of roadway swinging out of control and falling onto the homes, to the screams of onlookers.

Several buildings were practically flattened and rescue workers were searching for anyone trapped underneath, Dutch broadcaster NOS reported.

Emergency responders in Alphen aan den Rijn confirmed the accident on Twitter, but provided no details. Dozens of emergency vehicles were at the scene.

The bridge, over the Old Rhine river was being renovated after surveys in 2010 had shown that its steel superstructure was in poor condition. The renovation, which began in the spring was expected to last the whole summer.

The accident took place as the vast steel ramp that forms the central span of the swing bridge was lowered into place.