Barack Obama has launched a sharp assault on presidential rival John McCain and promised to reverse the economic failures of the past eight years and restore America's global reputation.

The first black presidential nominee of a major US party linked Arizona's Republican senator directly to President George W Bush and said the party's failed policies are responsible for a faltering US economy and a decline in US standing in the world.

As he accepted his party's nomination, Mr Obama told a flag-waving crowd of about 75,000 supporters in Denver's open-air football stadium: "On November 4, we must stand up and say, 'Eight is enough.'"

Mr Obama and running mate Joe Biden are set to hit the campaign trail after he delivered the biggest speech of his career on the 45th anniversary of Martin Luther King Jr's "I Have a Dream" speech - a landmark in the US civil rights movement.

He said Mr McCain was out of touch with the day-to-day concerns of Americans and had been "anything but independent" on key issues like the economy, health care and education.

"Now, I don't believe that Senator McCain doesn't care what's going on in the lives of Americans. I just think he doesn't know," said Mr Obama.

"Senator McCain likes to talk about judgment, but really, what does it say about your judgment when you think George Bush was right more than 90 per cent of the time?" he asked, citing Mr McCain's voting record in the US Senate.

"I don't know about you, but I'm not ready to take a ten per cent chance on change."

The televised acceptance speech by Mr Obama gave the first-term Illinois senator his biggest national audience until he meets Mr McCain in late September in the first of three debates. The two are running neck-and-neck in polls.

The speech included some of the most direct attacks on Mr McCain since the general election campaign started.

Meanwhile, Mr McCain has used a charm offensive in his latest attack on his rival.

In a televised ad, the senator congratulated Mr Obama for his performance during their last run-off and referred to the historical significance of appearance at the Democratic Party Convention.

But following the speech, the McCain camp accused the senator of being 'misleading'.

A spokesman said: "Americans witnessed a misleading speech that was so fundamentally at odds with the meagre record of Senator Obama.

"The fact remains, Obama is still not ready to be president."

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