Former US Secretary of State Hillary Clinton has formally entered the 2016 race for the White House in a bid to become the first woman US president.
She launched her campaign website on Sunday, telling Americans she wanted to be their "champion".
Mrs Clinton ran for the Democratic presidential nomination in 2008 but lost to Barack Obama.
The overwhelming Democratic favourite, she had been expected to declare her candidacy for months.
In a video on her website, Mrs Clinton declared: "I am running for president".
"Americans have fought their way back from tough economic times," she said, "but the deck is still stacked in favour of those at the top.
"Everyday Americans need a champion and I want to be that champion," she added.
The video features a number of Americans talking about their hopes and aspirations.
It ends with Mrs Clinton saying: "So I'm hitting the road to earn your vote because it's your time and I hope you'll join me on this journey".
Mrs Clinton's team said would spend the next few weeks building up grassroots support in the early Democratic primary states; she tweeted that she is on her way to Iowa.
She is expected to hold her first rally, officially kicking off her campaign, in mid-May.
Mrs Clinton has already had the backing of Mr Obama, who told a news conference at the Summit of the Americas in Panama on Saturday that she would make an "excellent president".
And her successor in the post of secretary of state, John Kerry, called her a "good friend", telling ABC's This Week programme she "did a terrific job of rebuilding alliances that had been shredded over the course of the prior years".
But the attacks from the Republicans have already begun.
Republican presidential contender Rand Paul criticised Mrs Clinton for her handling of a September 2012 attack on a US diplomatic compound in Benghazi, Libya, in which the US ambassador was among those killed.
He also said questions remained about funds received by a charity set up by Mr and Mrs Clinton.
Jeb Bush, former Florida Governor and brother of George W Bush who is also considering whether to stand as a Republican candidate, tweeted: "We must do better than Hillary."