Two cranes have collapsed on buildings in the Netherlands injuring at least 20 people, media say.

The cranes were employed to restore the Juliana Bridge in the central Dutch town of Alphen aan den Rijn.

Amateur video showed one crane trying to hoist a section of the Juliana Bridge and collapsing under the weight, bringing the second crane down with it.

Dutch media said rescue workers were in contact with at least one person trapped in the rubble.

Images on Dutch media showed a row of crushed buildings.

The images and video were carried on the De Volkskrant website.

The website said a Dutch team normally used for earthquakes and other disasters had been sent to the scene.

A large number of people were at the site watching the bridge operation when the accident happened.

Ton Koot, a spokesman for the local fire brigade, told NOS that heavy machinery had been moved in to help with the rescue work.

He had no more information on the condition of those injured.

One of the two cranes, linked together on a pontoon, was lifting a vast steel ramp that forms the central bridge span when it toppled, taking the other crane with it a few seconds later.

Amateur video showed people jumping from the pontoon, with screams heard in the background.

NOS said that the cranes had hit two shops.

A survey in 2010 had shown the bridge over the Rhine to be in poor condition. Its renovation began this spring and was expected to take all summer.