At around 5.40pm yesterday evening, police received reports of concerns for a man, aged 40, on a bridge at Park Square roundabout.
As officers were en route to the incident, the man had fallen from the bridge at around 5.45pm. He was air lifted to hospital where his condition is described as life-threatening, police have said.
An eyewitness told The Star they saw three people trying to hold back the man from the bridge railings but couldn't stop him from falling.
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A number of roads were closed in the surrounding area and reopened at 6.35pm, with the exception of Commercial Street that remained closed until 7.30pm.
A South Yorkshire Police spokesman said: "Police are supporting the man’s family and members of the public who witnessed the incident.
"If you were in the area at the time or were also witness to the incident, please call contact us."
If you have any information, call police quoting incident number 822 of 29 March 2017.
If you are in distress or know someone who might be, help is available. Contact Samaritans for free on 116 123.