US President Barack Obama has described Sunday morning's gay nightclub shooting in Orlando, Florida, as "an act of terror and an act of hate".
Americans were united in grief, outrage and "resolve to defend our people", he said.
Omar Mateen, 29, killed 50 people and wounded 53 at the Pulse club before being shot dead by police.
The so-called Islamic State group has said it was behind the attack, but the extent of its involvement is not clear.
A statement on its affiliated Amaq news agency said that an IS "fighter" was responsible.
NBC News reported that Mateen had called the emergency services before the attack and swore allegiance to IS.
The gunman was killed in an exchange of fire after taking hostages at the club.
The attack is the worst mass shooting in recent US history.
Six of the victims have so far been named - Edward Sotomayor Jr, Stanley Almodovar III, Luis Omar Ocasio-Capo, Juan Ramon Guerrero, Eric Ivan Ortiz-Rivera and Peter O. Gonzalez-Cruz.
A nationwide moment of silence will be held at 18:00 local time (22:00 GMT).
In an apparently unrelated event, a heavily armed man was arrested in southern California after saying he was on his way to a gay pride event in Los Angeles.
'Get tough and smart'
Condemning the attack, Mr Obama said the "brutal murder of dozens of innocent people" was a further reminder of how easy it was to acquire a deadly weapon in the US and shoot people.
"We have to decide if that's the kind of country we want to be," said the president, who has campaigned for stricter gun laws. "To actively do nothing is a decision as well."
"It was an act of terror and an act of hate," he said, adding that it was a heartbreaking day for the LGBT community.
An attack on any American was an attack on all, he added, saying: "We are united in grief, in outrage and resolve to defend our people."
Mr Obama has ordered flags on federal buildings to be flown at half mast until sunset on Thursday.
Later the White House said Mr Obama had postponed a joint appearance with the Democratic Party's presumptive nominee for president, Hillary Clinton.
Meanwhile her Republican rival, Donald Trump, said Mr Obama should step down for refusing to use the words "radical Islam" in his remarks.
"If we do not get tough and smart real fast, we are not going to have a country any more," he said.
Mateen, a US citizen of Afghan descent who was reportedly born in New York and lived in Florida, was not on a terrorism watch list.
However, officials revealed that the FBI had twice interviewed him in 2013-14 after he made "inflammatory remarks" to a colleague before closing their investigation.
The death toll of 50 means that the Orlando attack surpasses the 2007 massacre at Virginia Tech which left 32 people dead.
Another 53 people were injured in the shooting.
States of emergency have been declared in the city of Orlando and surrounding Orange County.
Security was boosted for the last day of the Capital Pride LGBT festival in Washington DC, which included a moment of silence for the Orlando victims at 13:00 local time.
A gay pride parade has also been taking place in West Hollywood, Los Angeles. The city's mayor said police in the nearby city of Santa Monica had arrested a heavily armed man who said he was going to the parade.
He was later identified as James Howell from the state of Indiana. He is not known to have any connection to the Florida gunman.