In more official remarks, Trump spoke from the Diplomatic Reception Room, the same place where he addressed the country in the aftermath of the Congressional Baseball Game practice shooting, which only left the gunman dead.
This time there were many more victims, with the president first wishing them well in a 7:11 a.m. tweet.
"My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!" Trump has written.
Standing at the podium several hours later, he began by saying the country was joined together in "sadness, shock and grief."
"Last night, a gunman opened fire on a large crowd at a country music festival in Las Vegas, Nevada," Trump said, not identifying the suspect, who police say is 64-year-old Stephen Paddock.
"He brutally murdered more than 50 people and wounded hundreds more," the president said, calling the act 'pure evil.'
He assured the public that the FBI and the Department of Homeland Security were coordinating with local officials on the ground, as he thanked those first responders for their quick efforts for sniffing out the shooter, carrying out his act from the Mandalay Bay Hotel's 32nd floor.
"The speed with which they acted is miraculous and prevented further loss of life," Trump said. "To have found the shooter so quickly after the first shots were fired is something for which we will always be thankful and grateful."
"It shows that true professionalism is all about," the president added.
Trump then turned to the victims, noting how 'hundreds' of Americans are now in mourning of a loved one.
"A parent, a child, a brother or sister," he said. "We cannot fathom their pain. We cannot imagine their loss."
"To the families of the victims, we are praying for you and we are here for you, and we ask God to see you through this very dark period," Trump said.
"To the wounded who are now recovering in hospitals, we are praying for your full and speedy recovery, and pledge to you our support from this day forward."
"In memory of the fallen, I have directed that our great flag be flown at half-staff," Trump said.
The president noted how "in moments of tragedy and horror" the country comes together as one.
"Our unity cannot be shattered by evil. Our bonds cannot be broken by violence. And though we feel such great anger at the senseless murder of our fellow citizens, it is our loves that defines us today, and always will, forever," he went on.
Trump said in times like these, "I know we are searching for some kind of meaning in the chaos."
"The answers to not come easy," he said. "But we can take solace knowing that even the darkest space can be brightened by a single light, and even the most terrible despair can be illuminated by a single ray of hope."
Winding down his short speech, he told the country that he and first lady Melania Trump were praying for those Americans affected by "this terrible, terrible attack."
'We pray for the entire nation to find unity and peace,' he said. 'And we pray for the day when evil is banished, and the innocent are safe from hatred and from fear.'
Earlier Monday, White House Press Secretary Sarah Huckabee Sanders said the president had been briefed on the "horrific tragedy."
"We are monitoring the situation closely and offer our full support to state and local officials. All of those affected are in our thoughts and prayers," she said in a statement sent to reporters.
Later in the day Huckabee Sanders confirmed that Trump had spoken with Nevada Gov. Brian Sandoval, Las Vegas Mayor Carolyn Goodman and the sheriff.
"The president gave a beautiful message," Goodman said, noting that Trump has shown "great concern."
Later on Monday, Trump reconfirmed his trip to Puerto Rico, which will take place Tuesday, a day before he heads to Vegas.
"It's been amazing what's been done in a very short amount of time on Puerto Rico," Trump said from the Oval Office Monday.
"There's never been a piece of land, that we've known, that's been so devastated," Trump added. "The bridges are down, the telecommunications was non-existent and it's in very, very bad shape. The electrical grid was totally destroyed."
Trump has been criticized for his response to Hurricane Maria, getting into a public tiff with the Mayor of San Juan on Twitter this weekend, suggesting Democrats encouraged her to act "nasty" to the president and that Puerto Ricans "want everything to be done for them."
Now he has a new disaster on his plate.
Early Monday morning, a shooter took aim from the 32nd floor of the Mandalay Bay Hotel, just off the Las Vegas strip, and rained bullets on a crowd gathered at a country music concert across the street, killing at least 50 people and injuring 200 more.
The suspect is 64-year-old Paddock, police say, who had an 'arsenal' of weapons in his room, including at least 10 guns.
Paddock committed suicide at the scene.
As news of the shooting broke early Monday morning on the East coast, Eric Trump, President Trump's middle son, started tweeting before his dad.
"My thoughts and prayers are with the people of #LasVegas," Eric Trump wrote.
"It is days like this that we should stop and thank all the men and women in uniform, who sacrifice their own lives every day to help others," he said, paying respects to American police.
After the president made his initial reaction, Vice President Mike Pence chimed in.
"To victims, families & loved ones affected by this senseless violence in Las Vegas, Karen & I are praying for you & offering our love [t]he hearts & prayers of the American people are with you. You have our condolences and sympathies," Pence wrote.
