Orlando gay nightclub shooting: 50 killed, suspect is Omar Mateen
The gunman, named by officials as Omar Mateen, was killed after taking hostages at the Pulse club.
President Barack Obama said Americans were united in grief, outrage and "resolve to defend our people" after "an act of terror and an act of hate".
The attack is the worst mass shooting in recent US history.
A nationwide moment of silence will be held at 18:00 local time (22:00 GMT).
Officials said the killings were likely to be ideologically motivated, though there was no information that the gunman was associated with a particular group.
However, US Congressman Alan Grayson said it was "no coincidence" the attack happened in a gay club.
"It may be we've seen the commission of an awful hate crime," he added.