Investigators were looking into what sparked the shooting but were not able to get a detailed description of the shooter, though they believe it was a man and were going through surveillance video and other evidence, interim Police Chief Joseph Chacon said at a 4am news conference.
Two people have critical injuries and no deaths have been reported.
The police have few leads regarding the identity of the suspect.
The motive for the shooting remains unclear.
"I'm happy to report no one has died", Chacon said. Investigators are still talking to witnesses and viewing video sources. Several outlets described the post-shooting scene as "chaotic", with medical first responders taking four of the victims to the hospital while police and private vehicles transported the other nine. The ninth remained near the scene and was being treated there, according to the Austin Police Department. He said he could not elaborate on that detail.
"Security (on Sixth Street) is always pretty good down here because of the way that we cordon off the streets, block off the streets, using barricades and try to keep people safe that way", he said. Although the shooting happened during ROT Rally, the interim chief said the incident was not related.
"It was very hard to contain the scene, it was very hard for EMS to make their way into this crowd", Chacon told reporters. One person went to an urgent care facility with multiple gunshot wounds, Chacon said.
Austin Mayor Steve Adler tweeted, "The uptick in gun violence locally is part of a disturbing rise in gun violence across the country as we exit the pandemic".
He said the state Department of Public Safety is assisting in the investigation.
In the early hours of 1 January 2020, two people were shot in a bar on the same street, a man and woman, who were seriously injured, at Terminal 6. "No community should ever experience what occurred this morning in Austin", Kelly said in a statement.