At least 50 people have been confirmed dead in a shooting at a gay nightclub in Orlando, Florida, the worst mass shooting to hit the US in recent history.
The suspect has been identified as Omar Mateen, who was killed by police after shooting revellers at the Pulse club and holding them hostage for several hours. At least 53 people were injured in the attack.
As events unfolded, the Pulse nightclub had earlier said: "Please keep everyone in your prayers as we work through this tragic event."
The scale of the massacre has left America in shock and police suspect an ideological motive, with NBC News reporting that Mateen had sworn allegiance to the Islamic State prior to the attack.
President Barack Obama said America stood united in the face of "an act of terror and an act of hate", and would not give in to fear.
"In the face of hate and violence we will love one another," he said.
"We will stand united as Americans to protect our people, defend out nation and take action against those who threaten us.
"We must not give into fear."
The sense of shock has gone beyond America's shores, with Pope Francis expressing his "deepest horror and condemnation" over the attack.
Vatican spokesman Rev Federico Lombardi said the Pope was sorrowed by the "homicidal folly and senseless hatred", and that he joined the families of the victims and those injured in "prayer and compassion".
Francois Hollande, the President of France, which has itself suffered recent devastating terrorist attacks, condemned the attack and offered the "full support of France and the French with America's authorities and its people in this difficult time".
According to the BBC, Mateen was a US citizen of Afghan descent from the Florida town of Port St Lucie, but was not on a terrorism watch list.
In terms of fatalities, the attack surpasses the 2007 Virginia Tech massacre in which 32 people died.
Orlando Mayor Buddy Dyer said there was an "enormous amount of havoc" and "blood everywhere".
"Today we're dealing with something that we never imagined and is unimaginable," he said.
"Because of the scale of the crime I've asked the [Florida] governor to declare a state of emergency," he said.
"We're also issuing a state of emergency for the city of Orlando so that we can bring additional resources to bear to deal with the aftermath."