The barrage of gunfire from the 32nd-floor of the Mandalay Bay hotel into a crowd of 22,000 people came in extended bursts that lasted several minutes.
More than 525 people were injured -- some by gunfire or shrapnel, some trampled -- in the pandemonium adjacent to the Las Vegas Strip as police scrambled to locate the assailant.
At least a dozen people were in critical condition at University Medical Center in Las Vegas where the most seriously injured victims were taken, a spokeswoman said.
A retiree armed with multiple assault rifles strafed an outdoor country music festival in Las Vegas from a high-rise hotel window on Sunday, slaughtering at least 59 people in the deadliest mass shooting in modern US history before killing himself.
The barrage of gunfire from the 32nd-floor of the Mandalay Bay hotel into a crowd of 22,000 people came in extended bursts that lasted several minutes, as throngs of terrified music fans desperately cowered on the open ground, hemmed in by fellow concertgoers, while others at the edge tried to flee.
The preliminary death toll, which officials said could rise, surpassed last year's record massacre of 49 people at a gay nightclub in Orlando, Florida, by a gunman who pledged allegiance to Islamic State.
The dead in Las Vegas included a nurse, a government employee and an off-duty police officer.
Shocked survivors, some with blood on their clothing, wandered streets after the shooting, where the flashing lights of the city's gaudy casinos blended with those of emergency vehicles.
Police identified the gunman as Stephen Paddock, 64, who lived in a retirement community in Mesquite, Nevada.
Authorities said they believed he acted alone, though his motive was unknown.
The Islamic State militant group claimed responsibility for the massacre, but US officials said there was no evidence of that.
Police said Paddock had no criminal record.
The gunman killed himself before police entered the hotel room from where he was firing, Clark County Sheriff Joseph Lombardo told reporters.
Federal officials said there was no evidence to link Paddock to militant organizations.
"We have determined to this point no connection with an international terrorist group," Aaron Rouse, special agent in charge of the Federal Bureau of Investigation (FBI) field office in Las Vegas, told reporters.
The gunfire erupted as country music star Jason Aldean was performing on stage.
Video of the attack showed terrified crowds fleeing under rapid gunfire as the shooter took aim from a distance of around 1,050 feet (320 m).
"People were just dropping to the ground. It just kept going on," said Steve Smith, a 45-year-old visitor from Phoenix, Arizona.
"Probably 100 shots at a time," Smith said.
Trump said he would travel to Las Vegas on Wednesday to meet with victims, relatives and first responders.
"It was an act of pure evil," said Trump, who later led a moment of silence at the White House in honour of the victims.
The White House said on Monday that it was too soon after the attack to consider gun control policies.
The suspected shooter's brother, Eric Paddock, said the family was stunned by the news.
"We're horrified. We're bewildered, and our condolences go out to the victims," Eric Paddock said in a telephone interview, his voice trembling. "We have no idea in the world."
Las Vegas's casinos, nightclubs and shopping draw some 3.5 million visitors from around the world each year and the area was packed with visitors when the shooting started shortly after 10 p.m. local time (0400 GMT).