Democratic Senator Barack Obama has launched his presidential campaign with a speech in which he pledged to "build a more hopeful America".
He began his official campaign with a call for the Iraq war to end, saying US troops must withdraw by March 2008.
Mr Obama, 45, is considered by many to be the first African-American candidate with a realistic chance of winning.
He, along with Senator Hillary Clinton, is leading the race for the Democratic Party's nomination for the 2008 vote.
A large crowd of supporters braved the sub-zero temperatures in Springfield, Illinois to watch Mr Obama make his announcement.
He spoke to the crowd of his working life in the state over the last 20 years, first as a community worker, then as a civil rights lawyer and finally as a US senator.
He said it was the lessons learnt watching the daily struggles many faced that had ignited in him a desire for change.
"That is why, in the shadow of the Old State Capitol, where Lincoln once called on a divided house to stand together, where common hopes and common dreams still live, I stand before you today to announce my candidacy for president of the United States," Mr Obama said.
As he made the announcement the crowd cheered and chanted his name.
But having served just one term as a senator and with no experience of executive office, many have questioned whether Mr Obama's skills match his ambition.
As he declared his candidacy Mr Obama acknowledged that fact saying:
"I recognise that there is a certain presumptuousness in this, a certain audacity to this announcement. I know that I have not spent a long time learning the ways of Washington, but I have been there long enough to know that the ways of Washington have to change."
Mr Obama said the first priority was ending the conflict in Iraq.
"America, it's time to start bringing our troops home," he said. "It's time to admit that no amount of American lives can resolve the political disagreement that lies at the heart of someone else's civil war."
Mr Obama was elected to the US Senate after Congress voted to support President George W Bush in his decision to go to war in Iraq, but in 2002 made a speech opposing the conflict.
He went on to criticise the current administration for what he called a failure of leadership to address issues like America's dependence on oil and a failing educational system.
Mr Obama burst on to the national scene in July 2004 when he delivered a stirring keynote speech at the Democratic party convention.
His declaration that there was no white or black America, but a United States of America helped him win a seat in the Senate that year and subsequently set him on a fast track to vie for the White House.
Time magazine has dubbed Mr Obama "America's hottest political phenomenon" and US chat show host Oprah Winfrey urged him to announce his candidacy on her programme.
But instead he chose to launch his presidential campaign on the very spot where Abraham Lincoln once denounced the divisions caused by slavery.
However, unlike previous black presidential candidates, Mr Obama was not part of the civil rights movement, which correspondents say makes some African-Americans wary of him.
His mixed race heritage - with a white mother from Kansas, and a black father from Kenya - has led some observers to suggest that he is an African and an American, but not an African-American.
Though undoubtedly ambitious and charismatic, with relatively little national experience and formidable opponents, including Mrs Clinton, many question whether he can really secure the Democratic nomination, and whether he has the depth of policy to match.
Mr Obama has tried to answer critics in recent weeks, introducing a bill that calls for the phased redeployment of US troops from Iraq.