About 20 people were killed and at least 42 people were wounded when a gunman armed with an assault-style rifle, a handgun and possibly an explosive device opened fire inside a gay nightclub in Orlando, Fla., about 2 a.m., Police Chief John Mina said.

The police classified the attack as a “terror incident.” Local and federal authorities were investigating, and the F.B.I. set up a hotline for tips.

The police and the federal authorities said they were pursuing several angles in the terrorism investigation, both overseas and in the United States.

The gunman was not from the Orlando area and was organized and well-prepared, the police said.

The gunman was found dead inside the Pulse nightclub, one of the largest gay clubs in Orlando, about 5 a.m. after a shootout with officers who were attempting to rescue dozens of people who were being held hostage, said Chief Mina. He confirmed that the gunman had been killed by the police.

The chief said it was difficult to assess the number of clubgoers who had been killed because many of the bodies had not been removed.

The police were searching the club for other explosives that the gunman may have planted during his assault on Pulse.

The shooting began inside the club, the chief said, and continued outside when an officer working at Pulse attempted to confront him. The gunman then ran back into the club, resumed shooting and took hostages.

At 5 a.m., Chief Mina said, the police decided to attempt a rescue, detonating two explosives to distract the gunman and to help clear the club.

“With that advantage,” the chief said, nine officers moved into the club to confront the gunman. In the ensuing shootout, one of the officers was slightly wounded, saved by his Kevlar helmet, the authorities said.

“We rescued about 30 people,” Chief Mina said. During the rescue and shootout, officers with the SWAT team — using an armored vehicle — entered the club and hurried people to safety. Many of them had sought shelter in other sections of the sprawling club.

The survivors escaped under the cover of what the police called the two “discretionary explosions,” the authorities said.

The wounded were taken to three area hospitals. Most were taken to Orlando Regional Medical Center, the area’s chief trauma center, which is just three blocks from the club.

On its Twitter feed, the Police Department asked other residents to “stay away from area” and said that it was seeking support from local state and federal agencies. Later, employees in the area were asked to stay home if possible.

The club itself posted a message on its Facebook page about 3 a.m.: “Everyone get out of pulse and keep running.”

Customers streamed out of the club into a chaotic situation with little idea of where to go. “Cops were saying, ‘Go, go, clear the area,’ ” Christopher Hansen told an Orlando television station. “You don’t know who’s what and who’s where.”

As the sun came up, helicopter-shot video showed a situation that seemed to be under control, although windows on one side of the club appeared to be shot out.

Phone camera videos shot at the scene show police cars, ambulances and other emergency vehicles outside the club on South Orange Avenue in downtown Orlando. Streets around the the club were shut down for several blocks.

Dozens of officers, paramedics and firefighters from the Orlando police, the Orange County Sheriff’s Office, the F.B.I. and three area fire departments were called to the scene.

“This will have a lasting effect on our community,” Chief Mina said. “We are a strong community.”

In the hours immediately following the shooting, the local television stations broadcast camera phone videos that captured the chaos at and near Pulse.

One of the videos clearly captured the rescue attempt. Shots could be heard as men and women ran from the nightclub.

Some of the clubgoers told local television reporters that the music was so loud it was difficult, at first, to distinguish the shots from the pulsing beat.

More than 100 people were believed inside the club when the shooting began about 2 a.m.

Orlando Regional Medical Center was placed on lockdown after receiving the first of the victims. “Only essential workers are being allowed access into the building,” the hospital said in a statement.

“Out of an abundance of caution, Arnold Palmer Hospital and Winnie Palmer Hospital have also been placed on lockdown,” the statement added.

On Pulse’s Facebook page, club officials promised to post updates as they came in: “As soon as we have any information, we will update everyone.”

“Please keep everyone in your prayers as we work through this tragic event,” the nightclub post said. “Thank you for your thoughts and love.”

Officials at Orlando Regional Medical Center asked family members of victims and missing people to gather at the north entrance, where they would be escorted inside.

Local television stations broadcast interviews with people outside the club and the hospital, anxiously waiting to hear from people who they said were still inside.

WKMG and WESH broadcast interviews with the crowds milling outside the hospital. One woman said she rushed to the area after she got a call from her daughter, who said, “Please come get us and come get us now.”

One mother told CNN: “A girlfriend of my son called and said he got shot, he got shot and she was crying.”

She said she could not find her son, or even get information on where he had been taken. Some of the victims had been transferred to other hospitals “because this hospital was full,” she said outside Orlando Regional Medical Center.

The club, which calls itself “Orlando’s Latin Hotspot,” hosts an “Upscale Latin Saturdays” party on Saturday night with three D.J.s and a midnight show.