Police in York are searching the River Ouse and surrounding area in the search for man.
They are warning of disruption around the river as it is not know how long the search will take.
Certain parts of King's Staith and Queen's Staith will be closed to members of the public while the enquiries are ongoing.
There are no further details of the man available at this time.
Anyone who may have witnessed the incident or who has any information which can assist the investigation is urged to contact North Yorkshire Police on 101 - select option 1 - and pass information to the Force Control Room.
Police have begun a search of the River Ouse in York after after receiving reports that a man went into the river in the early hours of Thursday morning. North Yorkshire Police say they are searching the river and the surrounding banks in the city centre to try and locate the man.
Parts of King's Staith and Queen's Staith have been cordoned off. Officers say there will be significant activity around Ouse Bridge as efforts to find the man continue. Police say the man was reported to have entered the river but was not seen to leave the water.
The River Ouse Safety Group have issues a statement following the death of Megan Roberts and river searches for Ben Clarkson who's been missing since the weekend.
The group comprises:City of York Council
North Yorkshire Police
North Yorkshire Fire & Rescue
Yorkshire Ambulance Service NHS Trust
Canal & River Trust
Maritime and Coastguard Agency
York Boat Rescue
“The River Ouse Safety Advisory Group is made up of representatives from the emergency services, commercial boat operators and authorities across the region who work together to provide support and safety advice through the joint emergency plan which we maintain, develop and exercise.
“As a group we are determined to work with colleagues across the city to prevent river-related incidents where we can. We urge everyone who is out and about in the city centre to watch out for each other and stay together when heading home so that should an accident occur, there is someone to raise the alarm.”
A 16 year old boy has been run over while he was inside atent in Barlby near Selby** in North Yorkshire. Police were called out at 3.55am thismorning to a field where a group of teenagers were camping near York Road and the River Ouse.
A man who had approached the group and chatted with them before threatening to run over the tent, later returned in a white van. Most of the group managed to get out of the way, but the 16 year old boy who was still in the tent ended up with serious injuries.
He is being treated at York District Hospital.
This is a disturbing incident which has resulted in a teenager sustaining serious injuries. It is vital that we obtain as much information as possible of the events leading up to the incident.