Power changed hands traditionally and quietly behind closed doors in Buckingham Palace as Blair first called on Queen Elizabeth II to submit his resignation to end a decade in power, and Brown arrived soon after to be confirmed as the new prime minister.
"This will be a new government with new priorities," Brown told reporters outside his Downing Street office minutes later. "I've been privileged with the great opportunity to serve my country."
Brown, a 56-year-old Scot known for his often stern demeanor, beamed as he was applauded by Treasury staff before heading with his wife, Sarah, to the palace, and he smiled broadly when he emerged.
The incoming leader, who for many lacks Blair's charisma, must woo Britons by shaking off the taint of backing the hugely unpopular Iraq war. With promises of restoring trust in government, he is planning to sweep aside the Blair era after a decade waiting for the country's top job.
President Bush was the first world leader to offer his congratulations in a phone call soon after Brown's appointment, Downing Street said.
Blair, who led the Labour Party to three successive election victories, later resigned his seat in Parliament and was announced as envoy to the Quartet of Mideast peace mediators.
Earlier, an emotional Blair received a warm send-off in the House of Commons - from his opponents as well as members of his own party - after one final appearance at the weekly question time session.
"I wish everyone - friend or foe - well. And that is that. The end," he said.
Legislators rose to their feet and applauded as he left for his meeting with the queen. Some, including Foreign Secretary Margaret Beckett, wiped away tears.
Blair also used the session to say he was sorry for the perils faced by British troops in Iraq and Afghanistan, but he gave no apology for his decisions to back the United States in taking military action.
Blair expressed condolences to the families of the fallen, this week including two in Iraq and one in Afghanistan.
"I am truly sorry about the dangers that they face today in Iraq and Afghanistan," Blair said.
"I know some may think that they face these dangers in vain; I don't and I never will. I believe they are fighting for the security of this country and the wider world against people who would destroy our way of life," he said.
"Whatever view people take of my decisions, I think there is only one view to take of them (the troops): They are the bravest and the best," Blair added.
David Cameron, leader of the opposition Conservative Party, saluted Blair's achievements and wished him well.
"He has considerable achievements to his credit, whether it is peace in Northern Ireland, whether it is work in the developing world, which I know will endure," Cameron said.
"I'm sure that life in the public eye has sometimes been tough on this family. So can I say on behalf of my party that we wish him and his family well, and we wish him every success in whatever he does in the future."
Protestant firebrand Ian Paisley, the Northern Ireland cleric and legislator whom Blair persuaded to work alongside the territory's Catholic minority - achieving peace after decades of bloodshed - also paid tribute.
Blair faced "another colossal task" as a peace envoy, Paisley said, adding he hoped the success in Northern Ireland would be repeated.
Workers packed furniture and boxes into a van outside Blair's Downing Street home before he handed over power.
Brown will 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.
Few expected the dour former finance chief to be greeted with public enthusiasm. In fact, Brown's ascension was widely seen as a political gift for the more youthful Cameron.
Dozens stopped near Downing Street to watch the day of high drama, including 39-year-old secretary Patrick Lee.
"I'm just here for historical purposes," he said. "I took time off work just to watch."
Judith Brown, 25, a student from Belfast in Northern Ireland, said she came to see Blair.
"I think it's romantic," she said. "I'm really surprised the crowds are so small because I thought there would be, like, thousands here. I mean, it's the end of an era."
Blair's last full day in office brought an unexpected present - the defection of a Conservative legislator to his Labour party. The move put Brown in bullish mood and he will now weigh calling a national election as early as next summer.
Bush paid tribute to his ally by saying "Tony's had a great run and history will judge him kindly."
"I've heard he's been called Bush's poodle. He's bigger than that," Bush told Britain's The Sun tabloid in remarks published Wednesday.
Bush is thought to have been instrumental in winning Blair his new role as Mideast peace envoy.
Irish leader Bertie Ahern said Blair he told him his new role would be "tricky," but said he wanted to focus on peacemaking.
"He believes if you have hands-on, persistent engagement then you can have real progress," Ahern told Ireland state broadcaster RTE.
Brown has waited 13 years for this moment. Most keenly watched will be his policy toward Iraq, where the number of British troops has rapidly fallen this year.
Blair has 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.
"His hands, whilst not quite clean, are certainly not sullied," said Alasdair Murray, the director of CentreForum, a liberal think-tank. Brown can "portray it as Blair's war and differentiate himself."
Brown may sanction an inquiry on Iraq, similar to the U.S. Study Group, telling a recent rally that Britain needs to acknowledge mistakes made over the conflict.
In Europe, bridges have been built with German chancellor Angela Merkel and new French president Nicolas Sarkozy, but tensions are likely to emerge.
The succession ends a partnership at the pinnacle of British politics that began when Brown and Blair were elected to Parliament in 1983 - sharing an office and a vision to transform their party's fortunes.
It has been widely reported - but never confirmed - that the two agreed to a pact over dinner in 1994 - with Brown agreeing not to run against Blair for the Labour leadership following the death of then party chief John Smith.
In return, Blair reportedly vowed 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.
Although Brown, who was unopposed in a contest to select Blair's successor, is moving jobs, he won't be moving house.
He, his wife and two young sons already live in the private quarters at No. 10 Downing Street - the prime minister's official residence - having switched homes with Blair's larger family, who needed the roomier apartment next door in No. 11, the Treasury chief's official residence.
(Copyright 2007. The Associated Press. All rights reserved.)