People in Halloween costumes stood in the incident area as emergency personnel attended to casualties laying in the street a video posted on Instagram, verified by Reuters, showed.
The injured people were taken to hospital and are stable, local media said.
The shooting happened late at night within Orlando's entertainment district where hundreds of people were celebrating Halloween.
Orlando Police Chief Eric Smith told a press conference that the alleged gunman, a 17-year-old boy, had been arrested, CNN reported.
He is being interviewed by detectives, Chief Smith said.
"He pretty much walked into downtown, walked into the street and did what he did," local U.S. media Fox 13 Tampa Bay said, quoting the police chief.
The Orlando police department had previously said in an X post that it was conducting a shooting investigation in Downtown Orlando. The department did not immediately respond to a Reuters' request for comment.
