According to a press conference given by Dayton Mayor Nan Whaley, at least 26 others were injured, but no details about the victims were released.
The Dayton Police Department said nine people had been killed in the shooting which began in the Oregon District at 1:00 am local time.
The suspect, who was shot and killed by responding officers, has not yet been identified.
The shooter has not yet been named, but authorities have confirmed he was wearing body armour, had high capacity magazines and had extra magazines with him.
"Had this individual made it through the doorway of Ned Peppers with that level of weaponry, there would have been catastrophic injuries and a loss of life", Biehl said.
No officers were injured in the confrontation, Carper said.
Biehl says the.223-caliber rifle Betts used was ordered online from Texas and transferred to Betts at a firearms dealer in the area.
The Ohio shooting was the third major outbreak of USA gun violence, coming seven days after a teenager killed three people with an assault rifle at a food festival in Northern California before taking his own life.
In a Facebook post, the bar said, "We have police regularly staffed next to our business who engaged the shooter and neutralized the threat".
Bullet holes remained visible in the window there as people left flower bouquets in memorial in front of Ned Peppers and other bars. "And if we did not have police in the Oregon District and the thousands of people in the Oregon District enjoying their Saturday evening, what we could have had in this city", Whaley said.
"Officers acted. instantaneously and effectively ended this in 30 seconds", Police Chief Richard Biehl said during a news conference. "We can not remain idle and allow this level of carnage to ravage our communities".
The motive behind the shooting was not immediately clear, he added.
In addition to nine people killed, 27 were injured, according to authorities. "As bad as this is, it could have been much, much worse, as I think everyone will become aware of here as more information unfolds", CNN quoted Carper as saying. Miami Valley Hospital spokeswoman Terrea Little said 16 victims have been received at the hospital, but she couldn't confirm their conditions.
"We believe he was planning this or something like this for a while", the source says.
Gov. Mike DeWine issued his own statement, announcing that he ordered flags in OH remain at half-staff and offering assistance to Whaley and prayers for the victims.
Whaley tweeted, "I'm heartbroken. It's absolutely heartbreaking and I commend the courageous first responders who acted so quickly to save lives". Unlike many other news organizations, we have not put up a paywall.
Dayton Police previously said via Twitter that were "investigating an active shooter incident" in the city's OR district and asked residents to avoid the area.
A vigil will be held at 20:00 local time in the OR district where the attack happened, Dayton Mayor Nan Whaley announced.