Barack Obama has accepted the Democratic Party's historic nomination to run for president of the United States in front of a crowd of about 75,000 wildly-cheering people.
In an address to the party's national convention in Denver, he promised to do his best to keep alive the American dream of opportunity for all.
"America, we are better than these last eight years. We are a better country than this," he said.
Mr Obama is the first African-American to be nominated by a major US party.
In his speech, Mr Obama promised to reverse the economic downturn afflicting the US and restore the nation's standing in the world.
"This moment - this election - is our chance to keep, in the 21st Century, the American promise alive," he said.
He also attacked the record of the Bush administration and his Republican rival for the presidency, John McCain.
Mr Obama criticised Mr McCain as out of touch with the concerns of ordinary Americans and said he had failed to help them on issues such as the economy, health care and education.
He also stressed that he would call for the withdrawal of troops from Iraq, whereas Mr McCain stood "alone in his stubborn refusal to end a misguided war", he said.
In a final rallying call, Mr Obama recalled the message of Martin Luther King, who - 45 years ago to the day - gave his "I have a dream" speech in his historic march on Washington.
"America, we cannot turn back. We cannot walk alone. At this moment, in this election, we must pledge once more to walk into the future," he said.
Joined on stage by his family and presidential running mate, Joe Biden, Mr Obama was given a standing ovation by the crowds.
Former Vice-President Al Gore called on the Democrats to "seize this opportunity for change" and elect Mr Obama.
Linking Mr McCain firmly to the policies of President George W Bush, Mr Gore said it was vital that Americans changed course if they wanted to tackle a "self-inflicted economic crisis", protect the rights of every American and halt global warming.
On Wednesday, Mr Obama was resoundingly endorsed by former President, Bill Clinton. His wife Hillary Clinton has also given her backing to the Democratic candidate.
National conventions are often the first time voters start to pay attention to a presidential race. Opinion polls show many voters are still unfamiliar with Obama and concerned about his readiness for the job.