Hillary Clinton on Wednesday said she was “sorry”, she didn't win the election, adding “this is painful, and it will be for a long time.”

“This is painful and it will be for a long time but I want you to remember this: Our campaign was never about one person or even one election. It was about the country we love and about building an America that's hopeful, inclusive and big-hearted,” said Hillary.

The Democratic presidential candidate was delivering what her campaign billed as a concession speech to Republican Donald Trump after his upset victory in Tuesday's election.

Democrat Hillary Clinton conceded the 2016 US White House race to Republican Donald Trump on Wednesday and offered to work with the president-elect, who she hoped would be a successful leader for all Americans.

She also urged her assembled staff and supporters, deflated after recent national opinion polls indicated a good chance at victory, to continue to work for a better nation. Clinton urged supporters to keep an open mind on Trump and give him a chance to lead.

“Last night I congratulated Donald Trump and offered to work with him on behalf of our country. I hope that he will be a successful president for all Americans,” Clinton told hundreds of supporters and staff.

“This is not the outcome we wanted or we worked so hard for, and I'm sorry that we did not win this election for the values we shared and the vision we hold for our country,” said Hillary.

The former first lady, US senator and secretary of state said the election results showed the nation was deeply divided, but the voters had spoken.

Ashen-faced aides sat in the front row as supporters in the audience sobbed at the emotional event.