"Many lives were lost," Mayor Buddy Dyer said. "Tonight we had a crime that will have a lasting effect on our community."

The authorities refused to provide the exact death toll, but mentioned that some 20 people might have been killed in the shooting. Around 42 people were hospitalized, officials said at a press conference on Sunday.

A policeman first engaged a suspect at the entrance of the Pulse club after he started firing at around 2 a.m. local time. The shooter then entered the club, turning the firefight into a hostage situation, Orlando police chief John Mina said.

"At approximately 0500 the decision was made to rescue hostages that were in there," he said.

A SWAT team moved in and killed the attacker on the premises.

"The suspect is dead; he appeared to be carrying an assault-type rifle, a handgun, and some kind of device with him," Mina added.

The assailant appeared to have been prepared for the attack, according to the police. An FBI spokesman said that the shooting was being investigated as an act of terrorism.

Witnesses reported that the gunman used an automatic weapon.

Club visitor Jon Alamo said he was in the back of one of the club's rooms when an armed man came into the front.

"I heard 20, 40, 50 shots," Alamo said. "The music stopped."

Another patron, Rob Rick, said the incident took place around closing time on Sunday morning, with around 100 people still inside.

"Everybody was drinking their last sip," he said.

When the shooting occurred, around 2 a.m. local time (0800 GMT), the club, on Facebook, warned people to "get out of pulse and keep running."

Multiple emergency vehicles were reported to have been dispatched to the scene, including the Fire Department's bomb squad and hazardous material team.

The police said they conducted a controlled explosion in the area.

US media reported that between seven and 20 people had been injured and that the suspected shooter may have been carrying a bomb. People at the club made similar reports on social media.

The incident comes less than a day after a gunman killed American singer Christina Grimmie after an Orlando concert. Her assailant committed suicide afterwards.

dj/sms (AFP, dpa, Reuters, AP)