It is still not known how many people, if any, are trapped in the rubble, the town`s vice-mayor said.
Images on Dutch media showed a row of crushed buildings.
Witness Dick van Smirren, who filmed the incident, told the NOS national broadcaster: "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."
De Volkskrant newspaper 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, said that heavy machinery had been moved in to help with the rescue work.
He said earlier reports of up to 20 people being injured were based on those who might be in the buildings, and that only one person had so far been taken to hospital.
Town vice-mayor Kees van Velzen said: "We will start looking for people under the rubble - the victims are our first priority. But there is a collapsing danger, so first we have to check stability of the homes and cranes.
"At the moment we don`t know how many victims there are or if there are any."
The two cranes, linked together on a pontoon, were lifting a vast steel ramp that forms the central bridge span when they toppled.
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.
More about:
