London - Conservative leader David Cameron walked into No. 10 Downing St. on Tuesday night as Britain's new prime minister, ending five days of political limbo and 13 years of Labour Party rule after forging a historic coalition that spans the country's political spectrum.

The deal that brought Cameron to power in the wake of indecisive elections last week united the Conservatives and the Liberal Democrats in Britain's first coalition government since Winston Churchill's war cabinet in the 1940s. The unlikely marriage of the center-right and center-left parties, with divergent stances on such major issues as national security and immigration, led some analysts to question its durability. But after exchanging major concessions in the name of forming a "strong and stable government," the two parties were set for their first joint cabinet meeting today.

Cameron, a self-described conservative "modernizer" who has embraced gay rights and "green" policies, agreed to give the post of deputy prime minister to the Liberal Democrats' leader, Nicholas Clegg, a champion of the poor. The Liberal Democrats, who actually lost seats last week, also won four other cabinet posts and prevailed in their demand for a referendum on broad electoral reform that could aid their party in future elections.

The agreement resolved the uncertainty over Britain's leadership after voters gave the Conservatives more votes and more seats than any other party, but not the majority they needed to govern alone. The Liberal Democrats finished third, behind outgoing Prime Minister Gordon Brown's Labour Party, which suffered its worst defeat in 80 years.

Brown tendered his resignation to Queen Elizabeth II even before the Conservatives and Liberal Democrats had completed their negotiations. Brown's surprise announcement set off a dizzying chain of events that forced Cameron to jump into his silver Jaguar to follow Brown in meeting with the queen.

In a swift handoff that contrasts dramatically with the long transition between presidential administrations in the United States, Cameron spoke for the first time as prime minister eight minutes after leaving Buckingham Palace. He paid tribute to Brown, then spoke frankly about the economic problems and inter-party tensions facing the new government.

Noting the challenges entailed in coalition government, Cameron said, "Nick Clegg and I are both political leaders who want to put aside party differences and work hard for the common good and for the national interest."

President Barack Obama called Cameron, Britain's youngest prime minister in nearly two centuries, to congratulate him shortly after the Conservative leader took over.

In a statement issued by the White House, the president said he told Cameron the United States "has no closer friend and ally" than Britain and restated his commitment to maintaining their special relationship.

The new government is unlikely to bring major changes in transatlantic ties. Though Clegg has sought a more independent line with Washington and vowed to bring British troops home from Afghanistan within five years, the Liberal Democrats have not advocated an immediate withdrawal, and the key post of foreign secretary went to William Hague of the Conservatives, who are staunch backers of the special relationship.

The deal struck Tuesday marked the end of an era of Labour rule that started with the party's landslide victory under Tony Blair in 1997. Blair's popularity dropped sharply over his support for the Iraq war, and three years ago he handed power to Brown, his longtime partner and often bitter rival.

Brown resigned a day after his party, in a last effort to stay in power, began talks of its own with the Liberal Democrats with a view to forming a "progressive alliance." In an emotional statement outside the prime minister's residence, Brown told the British public, "Thank you, and goodbye."

On Monday, knowing that Clegg would not agree to a deal that would keep him in power, the 59-year-old Scot offered to resign in the fall, allowing another Labour leader to take the prime minister's job then. The offer fell short. Talks between Labour and the Liberal Democrats broke down Tuesday, and Brown, stunning his rivals, opted to walk out of Downing Street before Cameron and Clegg hashed out a deal. His resignation left the Labour Party poised for a tough leadership fight - and what could be years in opposition.

"I have been privileged to learn much about the very best in human nature, and a fair amount, too, about its frailties, including my own," Brown said. He added that he had loved the job - "not for its prestige, its titles and its ceremony, which I do not love at all. No, I loved the job for its potential to make the country I love fairer, more tolerant, more green, more democratic, more prosperous and more just."

With that, joined by his wife, Sarah, and their two sons, he got into a car and was driven to Buckingham Palace to offer his resignation to the queen.

Despite their victory, the parties making up the new coalition are the strangest of political bedfellows. Their leaders appear to share a measure of chemistry; Cameron, for instance, is generally seen as standing to the political left of his base and Clegg to the right of his. But far-right Conservatives and fiercely progressive Liberal Democrats were grumbling Tuesday about an unholy union that some analysts say may not survive the five-year term ahead.

"Margaret Thatcher would have kittens," the Conservative-leaning Telegraph newspaper declared about the prospect of a deal.

To forge an agreement, both sides made major concessions likely to test the loyalty of their bases. The Liberal Democrats conceded on Conservative plans to begin slashing Britain's yawning budget deficit this year and signed on to a plan to keep and upgrade Britain's nuclear deterrent.

The Conservatives, meanwhile, agreed to offer tax relief to some of Britain's poorest citizens and scrap a plan to offer inheritance tax breaks to some of its wealthiest.

Yet sharp differences remain in other areas, particularly on the issue of relations with Europe. The Conservatives have vowed more independence from the European Union, a position the Liberal Democrats strongly oppose. As part of the deal, Clegg reportedly agreed not to press for a switch from the pound to the euro during the term of the coalition, but broader ties with Europe still appeared set to be a lightning rod.