US election 2016: Clinton hails 'milestone for women'
"Thanks to you, we've reached a milestone," she told cheering crowds at a rally in New York.
She hailed "the first time in our nation's history that a woman will be a major party's nominee".
Earlier, Mrs Clinton won the Democratic primary in New Jersey, cementing her hold on her party's nomination.
She went on to win South Dakota and New Mexico, while her rival Bernie Sanders found victory in the Montana and North Dakota caucuses.
Six states have been voting in primaries on Tuesday but the race in California will count the most.
Mr Sanders had been hoping for a win in that state but early results indicated a significant lead for Mrs Clinton.
As the votes were counted, he told supporters he would remain in the race and contest the remaining primaries, despite acknowledging it was a "very, very steep fight".
Mr Sanders aims to sway super delegates to support him instead of Mrs Clinton at the party's convention in July, but commentators say the Vermont Senator is unlikely to succeed in his bid for the nomination.
The AP news agency reported on Monday that Mrs Clinton already had enough delegates to qualify as the Democratic nominee.
Source: bbc.com