Gordon Brown ended a decade-long wait to become prime minister Wednesday, stepping from Tony Blair's shadow with a vow to repair divisions over the unpopular Iraq war, prolong Britain's historic stretch of prosperity and set a sober new tone at home and overseas.

Blair departed to rousing applause and even some tears from lawmakers. He takes on a daunting role as envoy to the Quartet of Mideast peace mediators after presiding over a decade of rapid social change — and rancor at the Iraq invasion.

Brown, who first vied with Blair to lead the Labour Party in 1994, smiled broadly as he left a meeting at Buckingham Palace during which Queen Elizabeth II asked him to form a new government, the ceremonial transfer of power.

Having departed the Treasury — which he led for 10 years — in a modest car with his wife, Sarah, he swept back to Downing Street in a government limousine.

"I remember words that have stayed with me since my childhood and which matter a great deal today — my school motto — 'I will try my utmost,"' Brown told reporters massed outside his new office.

"This is my promise to all of the people of Britain and now, let the work of change begin."

Brown tightly grasped Sarah's hand to guide her through the famous black front door of the prime minister's Downing Street office. He will now draw his legislative agenda and prepare for an election he must call by 2010.

U.S. President George W. Bush was the first world leader to offer his congratulations in a phone call soon after Brown's appointment, Downing Street said.

Their 10-minute talk was "cordial and constructive," Brown's official spokesman Michael Ellam said. He declined to specify if they discussed Iraq.

Brown also held brief telephone chats with the leaders of France, Ireland, Germany and Italy. He also spoke with opposition Conservative chief David Cameron.

Blair, who won three successive elections for his Labour party, was visibly emotional during a final weekly questions session with legislators,

He said he was sorry for the perils faced by British troops in Iraq and Afghanistan, but gave no apology for his decisions to back the United States in taking military action.

"I wish everyone — friend or foe — well," Blair said before departing the chamber to cheers. "And that is that. The end."

Legislators gave Blair a standing ovation as he left the House of Commons chamber to offer his resignation to Queen Elizabeth II. Some legislators, including Foreign Secretary Margaret Beckett, dabbed at tears.

Blair later took a train to Sedgefield, in northern England, to visit officials in his constituency, which he had represented since 1983. He resigned as a member of Parliament, officials said, a situation that will trigger a by-election in Sedgefield.

Brown, meanwhile, must woo Britons by shaking off the taint of backing the Iraq war.

A small group of protesters — including families of soldiers killed in Iraq — were allowed through police lines onto Downing Street to greet Brown with a chorus of jeers.

"We asked him politely to come and speak to us; he didn't," said Rose Gentle, whose son Pvt. Gordon Gentle died in 2004. "If he has ignored us once then I think he will continue to ignore us."

Others observers, such as Judith Brown, a 25-year-old student from Belfast, Northern Ireland, wanted to wish Blair a fond farewell.

"I think it's romantic," she said. "It's the end of an era. I'm hoping he might shed a tear or two."

Brown will also seek to head off a challenge from a revived opposition Conservative party. Polls already point to a "Brown bounce," with one survey putting his Labour party ahead of its rivals for the first time since October.

He will announce appointments to senior Cabinet posts Thursday, Ellam said. The spokesman declined to discuss media reports that Foreign Secretary Margaret Beckett has been told she will be replaced.

Few expected the former finance chief to be greeted with public enthusiasm. In fact, Brown's ascension was widely seen as a political gift for the more affable Conservative chief, Cameron.

But Blair's last full day in office Tuesday brought an unexpected present — the defection of a Conservative legislator to Labour. The move put Brown in bullish mood. He will now weigh calling a national election as early as next summer.

Most closely watched will be Brown's policy toward Iraq. British troop numbers there have rapidly fallen during 2007.

Blair left his successor an option to call back more of the remaining 5,500 personnel by 2008 — an opportunity likely to be grasped by a leader with a national election to call before June 2010. Brown confirmed Wednesday number will soon fall to 5,000.

The succession of Brown ends a partnership at the pinnacle of British politics that began when he and Blair were elected to Parliament 24 years ago — sharing an office and a vision to transform their party's fortunes.

It has been widely reported — but never confirmed — that the two men agreed on a pact over dinner in 1994, with Brown promising not to run against Blair for the Labour leadership following the death of then party chief John Smith.

In return, Blair reportedly agreed to give Brown broad powers as Treasury chief and to step down after a reasonable time to give Brown a shot at the senior post.

Brown has finally won his chance. "This will be a new government with new priorities," he told reporters. "I have been privileged to have been granted the great opportunity to serve my country."