At least 50 were shot dead by a rampaging gunman inside a gay nightclub in Orlando in what is suspected to be a "well planned" terrorist attack.
The gunman, who was later identified as Omar Mateen, a 29-year-old American citizen of Afghan descent, was killed by law enforcement officers.
At least 53 were reportedly transported to hospitals for treatment after the Sunday morning shooting, with the police warning people to stay clear of the danger zone.
"The killer was armed with an assault rifle and a handgun and had some other devices that we are investigating. Multiple people are dead inside...I don't want to give a number," the Orlando police chief was quoted as saying by CNN.
"It appears he was organized and well-prepared," the police chief added.
Investigators are already following leads that indicate that the shooter had "Islamist leanings", but nothing concrete has emerged yet, another official said.
It was about 2 a.m. that shots were heard outside 'Pulse', a gay nightclub just south of downtown Orlando, after which the gunman ran into the premises.
"Everyone get out of Pulse and keep running," the nightclub posted on its Facebook page shortly after.
A hostage situation soon developed with the police forcing their way into the club using an armoured vehicle.
The gunman was eventually shot.
While President Obama did not directly address the situation, the candidates were quick to their keyboards.
