The man went into the water with another male to help the distressed children.
Hampshire Constabulary said they were called to the scene at 3.45pm after reports were made that the man had entered the sea at Tower Street to assist in the rescue of two children:
"Both children are now out of the water as is another man who also went in to save them," the spokesperson said. "The search for the first man is continuing in the waters around Camber Docks and Old Portsmouth, and involves the Coastguard helicopter and the Ministry of Defence police boat.'
A Solent Coastguard spokeswoman said its rescue teams, the Coastguard helicopter, Gosport lifeboat, Hampshire Constabulary and Royal Navy personnel were all involved in the operation.
She said the man was last seen in water which has 'very strong currents, deep channels, ferries in and out and all sorts of traffic.'
A witness who did not want to be named told Cambridge News that she believed an 'older man' had 'swum out to sea to rescue two boys'.
"It's terrible. The police are still down on the beach now. Unless you were on the beach at the time you would not have seen what was happening. I hope they find him," she said.
So do we - what an awful thing to happen. Let's hope this have-a-go-hero is found safe and well very soon.