Hillary Clinton has launched her second campaign to become America's first woman president.

After months of heavy hints, the former first lady, senator and secretary of state formally announced her campaign with a social media blitz on Sunday.

Sixty-seven-year-old Mrs Clinton tweeted her decision, first in English and then in Spanish, with a clear message that she is chasing the votes of what Americans call the "middle-class".

She wrote: "I'm running for president. Everyday Americans need a champion, and I want to be that champion."

Her tweets, in both of America's two big languages, were signed "H", signally that she personally sent them.

A blue letter "H" with a red arrow was quickly unveiled as her campaign logo.

Most American pundits had assumed Mrs Clinton would run after her team rented office space in the New York borough of Brooklyn.

But her decision to formally announce in an orchestrated social media campaign with tweets and Youtube videos clearly shows where she thinks the 2016 presidential election will be fought and won.

There was even a minor Twitterstorm on Sunday as an expected announcement was delayed.

Angry reporters for America's network TV news outlets complained about a perceived hold-up in the announcement until her campaign sent them pizza.

Mrs Clinton has the backing of sitting president Barack Obama, her opponent for the Democratic nomination in 2008.

Her campaign launch comes ahead of potential rivals to represent the Democrats in the 2016 election.

A spokesman for former Maryland governor Martin O'Malley said the Democrat was "seriously considering running for president, and he will make his decision regardless of what other people decide to do".

Mr Obama's vice president, Joe Biden, 72, has said he will announce whether he will stand for the president at the end of this summer.

However, another potential rival, New York's Democratic governor Andrew Cuomo, endorsed Mrs Clinton, and Mrs Clinton remains the overwhelming favourite to represent her party in the November 2016 poll.

As her campaign team emailed wealth donors on Sunday, the former first lady, after a life time either in office or close to office, made it clear she would campaign as the candidate who had not forgotten her roots in in a working family of the Chicago "middle-class".

In her first campaign video, which showed the stories of what she called "everyday Americans" before her own. Mrs Clinton said: "The deck is still stacked in favour of those at the top."

If she secures the Democrat nomination, Mrs Clinton could face a dynastic battle with Jeb Bush, son of President George Bush and brother of President George W Bush.

Mr Bush, a former governor of Florida, has still to confirm if he will stand but is one of a number of Republicans to set up exploratory committees.

Mrs Clinton's husband Bill succeeded George Bush as president in 1993 only to be replaced by George W Bush in 2001.

Two confirmed candidates for the Republican nomination are Ted Cruz and Rand Paul, who represent Texas and Kentucky respectively in the Senate.

Mr Paul, speaking ahead of Mrs Clinton's announcement, accused the former first lady of "grand hypocrisy" for taking money from Saudi Arabia while criticising Republicans on women's rights.

Mr Cruz responded to Mrs Clinton's tweet almost instantly with a video embedded in to a tweet of his own. "We are ready for Hillary. Hillary Clinton represents the failed policies of the past," he said."Does America want a third Obama term?"