Police were called to Bradshawgate just before 1am following reports of around 30 people fighting outside Shots Bar.
It is believed to have started when a man was thrown out of the bar.
An eyewitness, who wished to remain anonymous, said: “There was a mass brawl at 00.30 this morning with 40 people involved that started inside Shots Bar.
“The fight continued on to Nelson Square.
“I witnessed males and females fighting and one male took his belt off and began trying to whip people with it.
“People were running out of shots bar with glasses and bottles.
“Police and ambulance arrived and one person looked badly injured.”
Greater Manchester Police (GMP) confirmed that one man was arrested for public order offences.
Police also said that a 28-year-old man was the victim of an assault,but confirmed that no one has yet been arrested for that offence.
A spokesman for GMP said: “We were called at 0.48am and told that 30 people were fighting and there were ambulances on the scene. There was also mention of somebody with a belt swinging his arms around.
“When we arrived, one man was arrested for public order offences.
“One person also suffered a section 47 assault. He was punched numerous times and had minor injuries.”
It was one of several incidents police were called out to across Bolton, over the weekend.
Two men were arrested for affray on Friday evening, following a dispute in the Queen Anne pub car park, in Junction Road.
Police were called just before midnight, after a concerned passer-by reported a violent disturbance
According to Police a Volgswagen Golf was badly damaged on the car park and a large group of men were involved in a dispute — recorded as an affray.
There had also been reports of a stabbing at the pub, with a witness saying somebody ran from the scene bleeding, but police confirmed there was no stabbing.