DENVER - Sen. Barack Obama, the first African American to lead a major-party ticket, accepted the Democratic nomination for president Thursday night, sharply criticizing Republican John McCain and casting the election as "our chance to keep, in the 21st century, the American promise alive."

"America, we cannot turn back, not with so much work to be done, not with so many children to educate and so many veterans to care for, not with an economy to fix and cities to rebuild and farms to save, not with so many families to protect and so many lives to mend," Obama told a roaring throng his campaign estimated at more than 84,000 packed into Invesco Field at Mile High. "America, we cannot turn back."

In a speech filled with policy specifics and some of the toughest swipes he has taken at his opponent in the campaign, Obama took on the sharp criticisms the GOP has leveled against him in recent months and at the same time exhorted the nation to look beyond politics as usual.

"If you don't have a record to run on, then you paint your opponent as someone people should run from," he said of his rival. "You make big elections about small things."

He acknowledged that a vote for a mixed-race candidate named Barack Obama may be a leap for many.

"I realize that I am not the likeliest candidate for this office. I don't fit the typical pedigree, and I haven't spent my career in the halls of Washington," he said. "But I stand before you tonight because all across America something is stirring. What the nay- sayers don't understand is that this election has never been about me. It's about you."

Obama's speech, nearly 45 minutes long, was the culmination of a four-day Democratic convention designed to unite his party, put more detail behind the slogan of "hope" and regain the momentum of a campaign slowed by sustained fire from his foes.

He accepted the nomination on the 45th anniversary of the Rev. Martin Luther King's "I Have a Dream" speech, and his campaign played to the historic moment. The promise of America, that hard work, equality and freedom can lead to boundless opportunities, "brought Americans from every corner of this land to stand together on a Mall in Washington, before Lincoln's memorial, and hear a young preacher from Georgia speak of his dream," Obama intoned.

He also emphasized the more traditional aspects of his biography: his Kansas-born grandfather who fought in World War II, his grandmother who lifted herself up from the secretarial pool to become a midlevel bank manager, and his mother, who pushed him hard, instilled her values in him, then died young.

The mood in the stadium bordered on ecstatic. The line to get in stretched over two miles, under a cloudless sky, while inside the stadium, volunteers registered voters, phoned supporters and organized the faithful. After Obama's speech, fireworks pierced the Denver night sky and confetti rained down on the field.

Even McCain acknowledged what he called "truly a good day for America" with an advertisement before and after the Democrat's address, saying: "Too often the achievements of our opponents go unnoticed. So I wanted to stop and say, congratulations. ... Tomorrow, we'll be back at it. But tonight, Senator, job well done."

But when the event was over, the battle resumed in an instant.