A search mission was under way involving the police and fire crews after the man "jumped" into the River Brun.
The incident happened at 4-30am today (Sunday) at Burnley town centre.
A spokesman for Lancashire Constabulary said: "We initially received a call after reports of an alleged assault. As we arrived on the scene armed with torches and assisted by fire crews, a man wearing a black T-shirt and jeans was spotted walking along the river bank before jumping in the water.
"Water rescue teams carried out searches over 400 metres of the river, but were unable to find the man. We believe, because the water was very shallow, the man had run off somewhere."