In the outpouring of solidarity similar to that seen after the Paris and Brussels attacks, political and religious leaders lined up to condemn the worst mass shooting in US history and to reject homophobia.
US President Barack Obama condemned the shootings in the Florida city as "an act of terror and an act of hate" and ordered flags at half-staff.
Pope Francis voiced shock at the "homicidal folly and senseless hatred" of slain gunman Omar Mateen, who gunned down 49 people at a gay nightclub in an attack claimed by the Islamic State group.
"Paris is with Orlando," the mayor of the French capital, which is still recovering from the November jihadist attacks in which 130 people were killed, tweeted along with a rainbow-coloured heart.
On Sunday, the spire of One World Trade Center in New York was already dyed red, orange, yellow, green, blue and purple, as was a sign in giant letters spelling Toronto in a square in the Canadian city.
Social media were flooded with messages condemning the attack using anti-homophobia hashtags #loveislove or #lovewins. Some posted images of a black ribbon or a ribbon combining the US and rainbow flag colours.
Following the latest in a string of mass gun killings in the United States that have claimed more than 1,000 lives in the past two years, Obama said the FBI was "appropriately investigating this as an act of terrorism".
"Although it's still early in the investigation, we know enough to say that this was an act of terror and an act of hate," he said.
"Although such deadly attacks cause profound sadness in us, we are resolved to continue with our open and tolerant lifestyle," she told German television on the sidelines of a visit to China.
While most leaders avoided polemical statements, the foreign minister of Lithuania, Linas Linkevicius, tweeted that the attack posed a "challenge for us all & #Muslims of the world.