Early exit polls had earlier provided optimism to the Clinton camp
Donald Trump looks on course for the White House after he claimed victory in the key states of Florida and North Carolina.
Pollsters, markets and bookmakers are now expecting Mr Trump to become the 45th president of the United States - but no call has yet been made.
Hillary Clinton supporters can be seen crying at what was supposed to be her victory rally in New York, while across town a carnival atmosphere is building at the Trump event.
The former First Lady's tweets have also started to take on a more sombre tone.
"This team has so much to be proud of," she wrote. "Whatever happens tonight, thank you for everything."
Early exit polls had provided optimism to the Clinton camp, but shortly after 2am (Irish time) the momentum began to shift as it emerged Mr Trump was edging ahead in a number of key states.
Key swing states of Ohio, Florida, Iowa and North Carolina all went to the billionaire as the results poured in.
Mrs Clinton has taken Virginia and Colorado, and looks set to win the popular vote across the nation - but it is electoral college votes that count.
World markets began to respond as the US map turned red, with falls reported across Asia and the Mexican peso dropping to a record low against the dollar.
The victor will not be inaugurated until 20 January. Barack Obama will remain in place until then.
Here's how the results stand:
Trump - 244 Electoral Votes
Alabama, Arkansas, Florida, Georgia, Idaho, Indiana, Kansas, Kentucky, Louisiana, Montana, Mississippi, Missouri, Montana, Nebraska, North Carolina, North Dakota, Ohio, Oklahoma, South Carolina, South Dakota, Tennessee, Texas, Utah, West Virginia, Wyoming
Clinton - 215 Electoral Votes
California, Colorado, Connecticut, Delaware, Hawaii, Illinois, Maryland,Massachusetts, New Jersey, Nevada, New Mexico, New York, Oregon, Rhode Island, Vermont, Virginia, Washington, Washington DC