Barack Obama has accepted the Democratic Party's historic nomination to run for president of the US in front of a crowd of some 75,000 people. In an address at the party's national convention in Denver, he promised he would do his best to keep alive the American dream of opportunity for all. "America, we are better than these last eight years," he told cheering crowds. "We are a better country than this." Mr Obama is the first African-American to be nominated by a major US party. In his speech at Denver's Invesco stadium, Mr Obama promised to reverse the economic downturn afflicting the US and restore the nation's standing in the world. "We are here because we love this country too much to let the next four years look just like the last eight years," he said. He also attacked the record of the Bush administration and his Republican rival for the presidency, John McCain. "This moment - this election - is our chance to keep, in the 21st Century, the American promise alive." 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. "I will restore our moral standing, so that America is once again that last best hope for all who are called to the cause of freedom, who long for lives of peace, who yearn for a better future," he said. He rejected criticism by the McCain campaign that he is a "celebrity", pointing to his family's past financial hardships, and said his rival should stop questioning his patriotism. 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," he said. "We cannot walk alone. At this moment, in this election, we must pledge once more to walk into the future." Joined on stage by his family and running-mate, Joe Biden, Mr Obama was given a standing ovation by the crowds. 'Not ready' Earlier in the day, Mr McCain ran a TV advert in which he congratulated Mr Obama on the historic nature - and date - of his nomination, saying it was "truly a good day for America". The political truce was short-lived, however, with a spokesman for the McCain campaign issuing a statement following Mr Obama's address that dismissed his words as "misleading". "Tonight, Americans witnessed a misleading speech that was so fundamentally at odds with the meagre record of Barack Obama," spokesman Tucker Bounds said. "The fact remains, Barack Obama is still not ready to be president." The BBC's Justin Webb in Denver says that this needed to be a serious speech by Mr Obama and it was.