Facebook Inc activated its "Safety Check" function for the first time in the United States after 50 people were killed when a gunman opened fire and seized hostages at a gay nightclub in Orlando, Florida.
The Safety Check, first introduced in October 2014, allows Facebook users to spread the word that they are safe in the wake of a natural disaster or a crisis, and allows searches for those who might be in the affected area.
"Waking up this morning, I was horrified to hear about the shooting in Orlando," Facebook chief executive Mark Zuckerberg said on his official account.
"My thoughts and prayers are with the victims, their families and the LGBT community."
A gunman armed with an assault rifle killed 50 people at a packed gay nightclub in Orlando on Sunday in the worst mass shooting in US history.
Police killed the shooter, who was identified asOmar Mateen, a 29-year-old Florida resident and US citizen.
The incident quickly emerged as a top trending topic on Facebook and Twitter, with hashtag #PrayforOrlando being one of the top posts throughout the day.
Last year, Facebook vowed to turn on the Safety Check more often during disasters in response to criticism that it enabled the function after Islamic State militant attacks on Paris and not a day earlier when a bomb killed at least 43 people in Beirut.