US president Barack Obama has called the Orlando mass shooting at a gay nightclub in which 50 people died an "act of terror" and an "act of hate".
The suspect, named as Omar Mateen from Port St Lucie, Florida, was armed with an assault-type rifle and handgun when he sprayed revellers with bullets at the popular venue, Pulse.
It is thought there were more than 300 people were inside the building at the time of the attack, which has now become the worst mass shooting in American history.
Other leaders have voiced their sadness at the events of the nightclub, including Pope Francis who expressed "horror" over the "homicidal folly" of the massacre, and French president Francois Hollande who said he "expresses the full support of France and the French with America's authorities and its people in this difficult time".
Buckingham Palace said the Queen sent a personal message to President Obama on Sunday night, saying: "Prince Philip and I have been shocked by the events in Orlando. Our thoughts and prayers are with all those who have been affected."
At a press conference Obama said, as Americans, they are "united in grief, in outrage and resolve to defend our people".
"The FBI is appropriately investigating this as an act of terrorism, and I directed that we must spare no effort to determine, what, if any, inspiration or association this killer may have had with terrorist groups," he added.
Obama praised the emergency service response, described the gunman as a person "filled with hatred" and said it was a "heartbreaking day" for fellow Americans who are lesbians, gay, bisexual or transgender.
"This is a sobering reminder that attacks on any American regardless or race, ethnicity, religion or sexual orientation is an attack on all of us and the fundamental values and dignity that define us as a country," he added.
"This massacre is therefore a further reminder of how easy it is for someone to get their hands on a weapon that lets them shoot people in a school, in a house of worship or in a movie theatre or in a nightclub."
During Obama's speech, Republican presidential hopeful Donald Trump took to Twitter to suggest the president should step down if he fails to mention radical Islamic terrorism.
The outspoken right-wing politician and businessman has faced criticism for posting a self-congratulatory tweet about the massacre in Orlando minutes after the horrific death toll was announced by police.
He faced wide condemnation on social media, including from former Labour shadow cabinet minister Chuka Umunna and former Star Trek actor and prominent gay rights activist George Takei.
The Democrat frontrunner, Hillary Clinton, posted on Facebook saying America needs to "redouble" efforts to defend the country from threats at home and abroad. In a move to reassure the LGBT community she said she is one of the millions of allies they have across the country.