A report was received at around 10.45pm that a man had been spotted floating in the River Bann.
Members of the public rescued the man from the river and administered first aid.
Police officers were on the scene within minutes and continued CPR until ambulance crews arrived to take the man to hospital where he remains in a critical condition.
An investigation is underway to determine the circumstances surrounding the man entering the water and his movements earlier in the evening.
A report that the man may have been assaulted in the area earlier in the evening at around 8pm is also being investigated.
A 23 year old man has been arrested in relation to the assault and is currently in custody helping police with their enquiries.
Detective Inspector Nicola Marshall said: “I would like to take this opportunity to thank the members of the public involved in the rescue of this man.
“Without their bravery and quick thinking it is highly likely that he would have died.
“I would also ask anyone who saw him earlier in the evening, witnessed an assault or who has any information that could assist us with our enquiries to contact CID at Lurgan on the non-emergency number 101.”