No injuries were reported.
Police spokesman Delrish Moss said the man, a cartoonist who "says he's been censored by the Herald," was carrying what appeared to be a machine gun. Moss did not identify the suspect by name.
Attorney Joe Garcia said the cartoonist, Jose Varela, called him a couple of times from inside the building. Varela was concerned about a conflict of interest at El Nuevo Herald, Garcia said.
"All that he wants people to know is that he wants the truth to come out," Garcia said. "I think he needs some time to work some things out."
The Miami Herald reported on its Web site that Varela walked into the sixth-floor newsroom, appeared agitated and demanded to see El Nuevo Herald Executive Editor Humberto Castello. About 12 to 15 employees were in the newsroom, employees said.
Rick Hirsch, The Miami Herald's managing editor for multimedia, remained in the building on the fifth floor with news staff trying to cover the story. He said he didn't know what might have upset Varela.
Hirsch said the building has security guards, but they are not armed.
"You have to have an ID to get in, but if this is somebody who came into the building regularly, there'd probably not be a problem," he said.
It was the second situation involving a gun at the newspaper building in the past year and half. In July 2005, former city commissioner Arthur E. Teele Jr. fatally shot himself in the Herald lobby after asking to speak with columnist Jim DeFede. Teele had been under investigation for corruption and had just been indicted by a federal grand jury on fraud charges.
DeFede was fired for recording his telephone conversations with Teele just before the shooting without the politician's permission.
The Miami Herald and El Nuevo Herald are separate newspapers but share a building and are both published by The Miami Herald Media Co. El Nuevo Herald is one of America's largest Spanish-language newspapers.