Dayton-based US Rep. Mike Turner tweeted that his daughter and a friend had just entered a bar across the street when the shooting began. They said he seemed to be acting alone, and that his motive is now unknown. A 21-year-old named as Patrick Crusius and from the city of Allen about 10 hours from El Paso is in custody.
Tianycia Leonard says she heard "loud thumps" she initially thought were people pounding on a dumpster.
This year alone there have been 251 mass shootings in the United States, resulting in more than 1,000 casualties.
More than two dozen people were injured in the attack at a busy shopping centre.
Montgomery County, Ohio, told CNN the assailant had been making his way to Ned Peppers from another bar when someone grabbed the barrel of his rifle, and he drew a handgun, but was then shot as police arrived.
She thanked first responders and said officials would share updates when they had more information.
Reports in local media and eyewitness accounts to NBC News suggest the shooting took place at or near Ned Peppers bar, in the heart of the district.
"The gunman acted alone with a ".223 high capacity" rifle with additional magazines, Dayton Mayor Nan Whaley said.
The suspect has not been identified.
The suspect had opened fire on the street firing "a long gun with multiple rounds". A family assistance centre was set up at the Dayton Convention Center.
In Dayton, a riverfront city of about 140,000 people in southwestern OH, a gunman dressed in body armor opened fire in a downtown district, unleashing carnage that could have been much worse if not for the rapid intervention of police.
Whaley said victims were receiving treatment at several local hospitals.
Earlier Dayton Police Department tweeted it was investigating reports of an "active shooter" in the city's Oregon District.
The suspected shooter's identity has not been released.
However, the FBI is investigating claims the alleged gunman posted a manifesto citing the mosque attacks not long before the attacks.
'Active shooter' reported in Dayton, Ohio
President Trump has been briefed on the shooting and praised law enforcement's speedy response in a tweet Sunday morning. Officers have advised people to avoid the area and said in a tweet: "This is a large scene and investigation".