Trump said he ordered flags on federal buildings to be flown at half-staff and added that he would visit Las Vegas on Wednesday to meet with first responders and victims.
The President closed his remarks by saying he prayed for "the day when evil is banished and the innocent are safe."
At least 58 people have been killed and more than 500 injured in the shooting, which began late Sunday night, police said. The suspected gunman was killed following what has now become the deadliest mass shooting in modern US history.
Thoughts and prayers from White House
Earlier Monday, Trump tweeted to offer his sympathies.
"My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting. God bless you!" Trump tweeted.
Trump has spoken with Nevada Gov. Brian Sandoval, Las Vegas Mayor Carolyn Goodman and Las Vegas Sheriff Joseph Lombardo
by phone, press secretary Sarah Sanders said. Earlier in the morning, the President was briefed on the situation by chief of staff John Kelly, she said.
Sanders called the mass shooting a "horrific tragedy" in her statement.
"We are monitoring the situation closely and offer our full support to state and local officials," Sanders said. "All of those affected are in our thoughts and prayers."
An administration official told CNN the President's trip to Puerto Rico to survey the hurricane response there, which was planned for Tuesday, was under review. Sanders said later in the morning that the visit was still on.
Attorney General Jeff Sessions released a statement saying he met with FBI Director Chris Wray and spoke with the Las Vegas sheriff.
Vice President Mike Pence issued a statement on Twitter, saying he and his wife were praying for the victims.
First lady Melania Trump tweeted her "heart and prayers" to the victims and those affected.
Ivanka Trump, the President's eldest daughter and a top adviser, tweeted "our collective hearts are breaking for the victims and their families."
Nevada politicians also responded Monday morning to the massacre, decrying the violence and offering thanks to first responders.
"Senseless, horrifying act of violence in Las Vegas tonight," Republican Sen. Dean Heller, who is from Nevada, tweeted. "Praying for all the victims & those impacted by the tragedy."
"Thankful for police and first-responders on the scene," Heller said.
Heller said he had spoken with Sandoval and state Attorney General Adam Laxalt.
Nevada's Democratic Sen. Catherine Cortez Masto offered prayers to the victims and thanks to the first responders on Twitter.
"Praying for all those affected by this senseless tragedy. Thank you to all the first responders. I will continue to monitor the situation," Cortez Masto tweeted.
Sandoval, a Republican, called the shooting a "tragic & heinous act of violence" and offered his prayers.
"A tragic & heinous act of violence has shaken the #Nevada family. Our prayers are w/ the victims & all affected by this act of cowardice," he tweeted.
Sandoval said in a statement that he would be in Las Vegas to meet with law enforcement and first responders and to console victims, their families and friends.
Members of Nevada's House delegation offered their thoughts as well.
Democratic Rep. Ruben Kihuen thanked the first responders in a tweet, and Rep. Dina Titus, also a Democrat, said in a statement that her "heart aches for the victims, their loved ones and our community."
Some call for gun control
As the morning wore on, politicians from around the country extended their sympathies to the victims of the shooting and thanks to first responders, while some Democrats issued forceful calls for gun control legislation.
Florida Republican Sen. Marco Rubio, whose state suffered a massive shooting in an Orlando nightclub last year, tweeted his prayers, and Connecticut Democratic Sen. Chris Murphy, who has kept up a drumbeat for gun control in response to the Sandy Hook Elementary School shooting in his state in 2012, tweeted, "Not again."
Murphy released a full statement later Monday morning calling for swift action on gun control.
"It is positively infuriating that my colleagues in Congress are so afraid of the gun industry that they pretend there aren't public policy responses to this epidemic," Murphy said, adding, "It's time for Congress to get off its ass and do something."
Murphy's press secretary, Laura Maloney, told CNN the senator was pushing a variety of measures, including legislation that would close loopholes in the background-check system, crack down on straw purchases and limit access to high-capacity magazines and assault weapons.
He went on to tweet about what he called his colleagues' "cowardice to act."
The state's senior senator, Democrat Richard Blumenthal, said, "The nation's conscience must be galvanized."
Massachusetts Democratic Rep. Seth Moulton said he would not take part in a moment of silence on the House floor as long as Congress did not act on gun control.
And New York Democratic Sen. Kirsten Gillibrand said her heart was with the victims, but "thoughts and prayers" were not enough.
"We must act to prevent this from happening again," Gillibrand tweeted.
Former Arizona Democratic Rep. Gabby Giffords, herself a victim of gun violence along with several others after an assassination attempt in 2011, said on Twitter that "no person should endure the horror Las Vegas experienced last night."
House Majority Whip Steve Scalise, who returned to the House floor last week after being seriously injured in a shooting at a congressional baseball practice in June, said he and his wife were praying for the victims of the "unspeakable violence in Las Vegas."
Scalise issued a statement following the President's remarks, saying he agreed with Trump and calling on people to respond to evil with "countless acts of kindness, warmth and generosity."
House Majority Leader Kevin McCarthy called the shooting "a senseless tragedy."
And House Speaker Paul Ryan ordered the flags over the Capitol lowered to half-staff in memory of the victims.
"America woke up this morning to heartbreaking news," Ryan said in a statement. "This evil tragedy horrifies us all. To the people of Las Vegas and to the families of the victims, we are with you during this time. The whole country stands united in our shock, in our condolences, and in our prayers."
The chamber's minority leader, Nancy Pelosi wrote on Twitter that she was "horrified and heartbroken."
Meanwhile, former President Barack Obama, who dealt with several mass shootings during his eight years in office
, tweeted: "Michelle & I are praying for the victims in Las Vegas. Our thoughts are with their families & everyone enduring another senseless tragedy."
Former Vice President Joe Biden also offered sympathies.
"Appalled by the senseless loss of life in Las Vegas. Jill and I hold all those affected and grieving in our hearts," he tweeted.
Former President Bill Clinton wrote, "Thinking of the victims and responders in Las Vegas. This should be unimaginable in America."
Hillary Clinton said she was grieving with Las Vegas, and the former secretary of state criticized the National Rifle Association for backing legislation that would make it easier to purchase silencers.
After the Scalise shooting, the House canceled
a hearing on that legislation, which has not yet come to a floor vote.
Foreign leaders voice support
Foreign leaders sent their condolences on the shooting Monday morning. United Kingdom Prime Minister Theresa May responded on Twitter, as did London Mayor Sadiq Khan.
"PM -- The UK's thoughts are with the victims and the emergency services responding to the appalling attack in Las Vegas," May tweeted.
Khan tweeted, "A deeply sad day for the city of Las Vegas. London sends our condolences to the victims and their families."
Canadian Prime Minister Justin Trudeau said, "Words fail this morning. The friendship & support of Canadians is with the victims in Las Vegas & the people of the US."