Former Secretary of State Hillary Clinton today announced her US presidential bid, emerging as one of the top contenders for the Democratic party nomination as she once again seeks to become America's first woman president.

The one-time first lady enters the race seven years after her bitter nomination defeat to President Barack Obama and the announcement is expected to trigger a donor deluge from a vast network of supporters who have long waited for her to officially enter the 2016 presidential race.

"I'm running for president. Everyday Americans need a champion, and I want to be that champion," Clinton made the announcement on Twitter.

In an accompanying video, Clinton pledged to be a champion for everyday Americans and their families. She said she believes this campaign is about voters, not her, and urged people to get involved in her campaign saying, "It's your time." The announcement came first in a tweet with an accompanying video and then in an email to her supporters. The video features middle class families talking about their lives and planning for what is ahead.