Hillary Clinton 'secures Democratic nomination' - AP
Hillary Clinton has clinched the Democratic nomination for US president after reaching the required number of delegates, an AP tally suggests.
The count puts Mrs Clinton on 2,383 - the number needed to make her the presumptive nominee.
She will become the first female nominee for a major US political party.
But rival Bernie Sanders said Mrs Clinton had not won as she was dependent on superdelegates who could not vote until July's party convention.
Mrs Clinton reached the threshold with a big win in Puerto Rico and a burst of last-minute support from superdelegates, AP said late on Monday night.
Superdelegates are party insiders who can pledge their support for a candidate ahead of the convention but do not formally vote for them until the convention itself.
AP's announcement came ahead of Democratic primaries on Tuesday in California, Montana, New Mexico, North Dakota, South Dakota, and New Jersey. Polling stations opened at 06:00 local time (10:00 GMT) in New Jersey. The last ones to close will be in California at 03:00 GMT Wednesday.
Analysis - Katty Kay, presenter, BBC World News
It has taken a long 227 years to get even this far.
George Washington was elected president of a newly independent America in 1789. Forty-three men later (42 of them white) Hillary Clinton makes history today by being the first female nominee for the White House.
So why don't I feel more excited?
The lack of exuberance may come from the fact that this has all been going on for so long.
We've really been reworking a version of the "first viable female candidate for the presidency" story since 20 January 2007, the first time Hillary Clinton declared her candidacy for the White House.
We're exhausted. We've run out of superlatives. We've overused every anecdote from the former first lady, former senator, former secretary's well-covered life.
A woman president would be new, Hillary Clinton is not.
At an appearance in Long Beach, California, shortly after the news broke, Mrs Clinton said: "We are on the brink of a historic and unprecedented moment but we still have work to do.
"We have six elections tomorrow and we're going to fight hard for every single vote, especially right here in California."
BBC North America editor Jon Sopel says the AP announcement could actually be a problem for Mrs Clinton, as it may depress turnout in Tuesday's primaries and creates the impression that she has only won because of the superdelegates and not the ordinary voters.
Sanders' California vow
Mr Sanders has vowed to stay in the race until the convention, and his campaign team said the Vermont senator would attempt to win back superdelegates.
His spokesman Michael Briggs said it was too early to call the contest.
"It is unfortunate that the media, in a rush to judgement, are ignoring the Democratic National Committee's clear statement that it is wrong to count the votes of superdelegates before they actually vote at the convention this summer," he said.
Mrs Clinton, a former secretary of state, New York senator and First Lady, leads Mr Sanders by three million votes, 291 pledged delegates and 523 superdelegates, according to AP's count.
She has won 29 caucuses and primaries to his 21 victories - and an estimated 2.9 million more voters have backed her during the nominating process.
That gives her a significantly greater lead over Mr Sanders than Barack Obama had over her in 2008 - he led by 131 pledged delegates and 105 superdelegates at the point he clinched the nomination.
"It's not complicated" - US media round-up
Philip Bump in the Washington Post unpicks the maths of the AP count, saying: "This is a subject of great consternation to rival Sen. Bernie Sanders, who... has repeatedly suggested that media outlets who point this out are irresponsibly abdicating their duty to their audiences. But it's not a complicated issue. If you accept that Donald Trump has clinched the Republican nomination, you should similarly accept that Clinton will seal her party's nomination on Tuesday."
Amy Chozick and Patrick Healy in the New York Times say: "Mrs Clinton must also work in the coming weeks to improve her own standing with voters, both with Mr. Sanders's hordes of young supporters and with a majority of registered voters who say they do not like or trust her."
Philip Elliot in Time quickly turned to the forthcoming presidential campaign, saying it will be "ugly". "Trump has invoked the scandals - and the faux-scandals -of the 1990s and has cast the current incarnation of Clinton as a "crooked" politician... He has shown little regard for her lifetime in the public eye... and has been devastating in his criticism. All which goads his massive audiences into rage against Clinton."
AP reports White House officials as saying that Mr Obama is preparing to endorse Mrs Clinton in the next few days, although the announcement would come after Tuesday's primaries.
Mr Obama telephoned Mr Sanders on Sunday, AP said. The contents of the call have not been revealed.
Speaking in San Francisco, Mr Sanders did not refer to the AP count, and promised supporters he would win in California: "As of today, we have won primaries and caucuses in 20 states across this country.
"And tomorrow, in the most important primary in the whole Democratic nominating process, we're going to win here in California."
On 26 May, Donald Trump passed the number of delegates needed to secure the Republican Party's presidential nomination, AP reported.