Bernie Sanders says he will continue to fight for the Democratic nomination.
Mr Sanders has thanked supporters during a speech in California where he says his supporters must continue to campaign to prevent Donald Trump from becoming president:
He said: "The American people, in my view, will never support a candidate whose major theme is bigotry, who insults Mexicans, who insults Muslims, women and African Americans.
"We will not allow Donald Trump to become president of the United States."
It comes after Sanders said he was "really disappointed" and "upset" that The Associated Press declared rival Hillary Clinton the presumptive Democratic nominee on the eve of primary elections in six states.
Mr Sanders expressed concern that the reports of Mrs Clinton's victory came the night before "the largest primary" - California - and was annoyed by the way it was done.
He said that it was based on what he described as "anonymous" commitments from superdelegates - the party insiders and Democratic officials who can vote for the candidate of their choice at the summer convention.
"They got on the phone as I understand it, and started hounding superdelegates to tell them in an anonymous way who they were voting for," he said.
"The night before the largest primary, biggest primary in the whole process, they make this announcement. So I was really disappointed in what The AP did."
The AP's delegate count is based on pledged delegates won in primaries and caucuses, as well as a survey of superdelegates. The AP only included superdelegates in its tally if they said they would unequivocally vote for Mrs Clinton or Mr Sanders at the convention.
According to the AP's count, Mrs Clinton secured the commitments of enough delegates to become the presumptive nominee on Monday night, the day before primary elections in California, New Jersey and four other states.