Barack Obama won the Democratic Party's presidential nomination Wednesday, becoming the first black nominee of a major U.S. party, as former rival Hillary Clinton moved for acclamation during a symbolic roll call vote.

After his running mate, Joe Biden, accepts his vice presidential nomination, Obama is expected to visit the Pepsi Center briefly to thank his supporters, a routine event at recent national conventions.

Over the din of roaring applause and booming music, House Speaker Nancy Pelosi, chairman of the Democratic National Convention, twice had to offer Obama's response.

"I have been asked to inform you that Senator Obama accepts the nomination and will deliver his acceptance speech tomorrow night at the fourth night of the convention to be held at Invesco Field," Pelosi said.

A raucous group cheered its way to the nomination in Denver after state delegates followed a scripted agreement to give a split rundown vote. The counting stopped when the vote came to New York, and Clinton called for a suspension of the roll call.

“Madame Secretary, I move that the convention suspend the procedural rules and suspend the further conduct of the roll call vote. All votes cast by the delegates will be counted and that I move that Senator Barack Obama of Illinois be selected by this convention by acclimation as the nominee of the Democratic Party of the United States," said Clinton, wearing a turquoise pant suit that matched the decor in the Pepsi Center convention hall. "Barack Obama is our candidate and he will be our president."

Earlier in the day, Clinton herself said she was voting for Obama, and made the announcement to suspend the roll call after hundreds of delegates continued to back her in spite of her releasing them.

Pelosi then sought a second to which the thousands of delegates collectively responded before a thunderous "aye" during a voice vote.

As the state tallies rolled in, Clinton didn't come close to the nearly 1,900 delegates she had won during the primary season. As Obama quickly closed in on the 2,210 delegates needed to seal the nomination, she had earned 341.5 votes by the time counting had ceased.

Obama, 47, and in his first Senate term, arrived in Denver Wednesday afternoon, and while the outcome of his nomination was assured, the mechanics had dominated the week.

"No matter where we stood at the beginning of this campaign, Democrats stand together today," declared Rep. Debbie Wasserman Schultz of Florida, a former Clinton supporter who delivered a nominating speech for Obama.

"We believe passionately in Barack Obama's message of changing the direction of our country," she said.

His formal acceptance speech Thursday night was expected to draw a crowd of 75,000 at Invesco Field, an achievement that Republicans sought to diminish by trying to brand him with the image of a celebrity without substance.

In response, Obama hopes to return the White House to Democrats by labeling Republican John McCain as more of President Bush. He has promoted the concept of change, including a pledge to pull U.S. combat forces out of Iraq in 16 months and to make health care available to all Americans.

Much of his strategy includes targeting western and southern states that have been Republican strongholds, but he is vulnerable in northern industrial states, where Clinton strongholds had been crucial to Democratic hopes. Obama, his wife Michelle and running mate Joe Biden and his wife, Jill, will embark on a bus tour of three of those states: Pennsylvania, Ohio and Michigan.

Obama's prospects in the Nov. 4 election are uncertain. McCain, a veteran senator and former prisoner of war in Vietnam, and his allies have been able to tar Obama for his lack of experience and judgment.

Obama also needs to unite a party fractured by the long and bitter primary campaign -- a process that received a boost Tuesday with an enthusiastic speech by Clinton, who said Obama is "my candidate and he must be our president."

Obama was expected to receive a similar support from her husband, Bill Clinton, who speaks to the convention Wednesday night. The former president has made little secret of his disappointment, or even anger, over his wife's primary defeat, but aides say he was planning a ringing endorsement of Obama as a leader ready to confront any challenge.

The difficulty in uniting the Clinton and Obama camps showed in the careful negotiations for the delegate roll-call that sealed Obama's nomination. Clinton, who won nearly 18 million votes and but could not overcome Obama's delegate total, had wanted the pro forma roll call as a cathartic moment for her huge bloc of supporters.

The compromise, that allowed her to be nominated and the votes to be counted through a partial roll call of states, provided a middle ground and a hope for united Democratic front.

Republicans hold their convention next week to anoint McCain as their candidate. He has not yet announced a running mate, but was expected to do so soon.

The Associated Press contributed to this report.