Up to 50 people have been killed in a shooting inside a gay nightclub in the Florida city of Orlando, say police.
Police said about 53 people were injured and taken to hospital.
Orlando Police say the shooter inside the Pulse Club was shot dead by officers who responded to reports of the shooting.
A bomb squad and hazardous material team are at the scene, and a controlled explosion was earlier carried out.
Relatives have been gathering at local hospitals desperate for news of their loved ones.
One woman said she had received a text message and phone call from her daughter inside the club, who said she had been shot in the arm.
Another witness told local TV reporter Stewart Moore that his friend was still locked inside the building with a gunshot wound.
Witnesses said they heard at least 20 gunshots as the incident unfolded.
Mr Moore earlier tweeted that he had been told by a source that more than 20 people had been shot.
Video footage being shared on social media showed dozens of emergency vehicles at the scene and people being treated on the pavements.
More than 100 people were reported to have been enjoying a Latin-themed night in the club, which calls itself the hottest gay bar in the city.
Ricardo Negron Almodovar told BBC News that the shooter opened fire at around 02:00 local time (06:00 GMT), as the club night was coming to a close.
"We heard rapid fire go off. In the room I was in, people went down to the floor. I wasn't able to see the shooter or people get hurt.
"At some point, there was a brief pause, and a group of us got up and went to the exit that leads to the patio area outside. We found an exit and after that... I just ran."
Another man, Anthony Torres, said he heard people screaming that others in the nightclub were dead.
The incident comes as Orlando is still reeling from the fatal shooting on Friday night of 22-year-old singer Christina Grimmie following a concert in the city.
She had been signing autographs when she was shot by 26-year-old Kevin James Loibl, who then killed himself.
It is not clear what Loibl's motive was.
-BBC / Reuters