The alarm was raised at around 9.45am today when a span of a new bridge, alongside Barton Bridge and the Manchester Ship Canal, plunged more than 80 feet.
Despite fears that some workmen may have been on the structure when it came down, early reports say there have been no casualties.
One eye-witness, working near to the scene, tweeted: “I’m working on a building site near the bridge and I heard a huge rumble that shook the whole building.”
Another wrote: “Fire and ambulances deployed. Massive noise and smoke like something fell.”
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The bridge, supported by four huge concrete towers, has been under construction for some weeks as part of a £50m project to reduce traffic congestion in the area.
A spokesman for the North West Ambulance Service said paramedic crews had been despatched to the scene, but only as a precaution.
There were no reports of any injuries, or people trapped. Everyone at the construction site had been accounted for.