The shooting happened around 5:15pm across from City Hall in Tranquility Park, near where the Occupy Houston protesters were gathered.
Detectives were still at the scene late Monday night investigating the scene of the shooting. They were identifying shell casings that littered Tranquility Park.
It all began hours earlier when a man armed with a rifle caught many people's eye in the park.
"First thing I hear is guns going off -- pow, pow, pow!" witness Reggie Green said.
"All the sudden, he turned around and shot into the water on the other side of the bridge. And that's when I yelled that's a real gun," witness David Loser said.
Police say the 21-year-old white male, who has been identified as Joshua Anthony Twohig, was armed with a .40-caliber assault rifle and fired off more than a dozen shots. The gunman also donning business attire.
"He is dressed in a black suit, with a black shirt and a white shirt," Loser said.
After being flagged down, two HPD bike officers responded and Twohig standing on this bridge. He refused to drop his assault rifle so police opened fire.
"They said they waited for a long as they possibly could before they were in fear of their safety and they both discharged their weapons several times," Houston Police Department Spokesman John Cannon said.
The gunfight played out in front of dozens of eyewitnesses and protesters with the Occupy Houston movement, which was set up just feet away.
"We don't know his motives; that being said he was not a member of the Occupy movement that we know of," Occupy Houston protester Dustin Phipps said.
Despite dozens of shots being fired in the downtown park that is across the street from City Hall, the gunmen was the only one injured.
"I'm glad that the gentleman did not point his gun at all these innocent people here," Loser said.
Police say at one point, Twohig put his weapon to his head, but officers took him down before he could shoot himself.
Twohig is charged with aggravated assault against a public servant. Police say Twohig suffered more than one gunshot wound and remains hospitalized at Ben Taub.
Houston police added that an increased police presence at Tranquility Park remains in effect Tuesday.