WASHINGTON - Barack Obama stood on the cusp of history Tuesday as hundreds of thousands of people streamed into Washington to witness his inauguration as the first black US president.

Millions of foreigners were also tuning in to see the son of a black Kenyan father and white American mother take office, in a generational power shift at a time of crippling economic crisis and challenges to US power abroad.

Obama and his wife Michelle, wearing a yellow dress, left the Blair House presidential guest house for a short trip across Washington's Lafayette Square to worship at St John's Episcopal Church, known as the "Church of Presidents."

The president-elect, 47, was then set to go to the White House for coffee with President George W. Bush, before the current and future president were to ride to Capitol Hill together for the momentous transfer of power.

A green Marine helicopter stood ready at the US Capitol to take Bush on the first leg of his journey back home to Texas.

On the National Mall, vast crowds stretched below the white dome of the US Capitol where the inauguration takes place, to the Washington monument, a towering white marble obelisk.

In the distance was the Lincoln Memorial, which Obama will see as he gazes out across the crowds to give his inaugural address, dedicated to the revered president who ended slavery.

New White House press secretary Robert Gibbs said the biggest speech yet of Obama's oratory-laden career would emphasize national unity to confront the sea of troubles bequeathed by Bush.

"I think you will also hear about a new era of responsibility," he told Fox News.

"Responsibility in the actions that government takes and responsibility by our financial institutions, some of whom got us into the economic mess that we are in today."

Obama's moment in history was also being closely watched abroad.

Pope Benedict XVI sent a message calling on the 44th president "to promote understanding, cooperation and peace" among nations.

French President Nicolas Sarkozy said he was eager to work with Obama to "change the world" while German Chancellor Angela Merkel wished him "the best of luck."

A BBC poll of people in 17 countries found an average of two-thirds believe Obama will improve frayed relationships between the United States and the rest of the world.

But with expectations running high at home and globally, Obama's team is pleading for patience as it confronts a groaning in-tray of challenges from Gaza to Guantanamo.

Vast, ethnically diverse crowds braved freezing cold to cram the National Mall mobbed streets leading onto the central thoroughfare, more than five hours before Obama was to take the oath of office.

Some 318,422 people jammed the Washington subway system early Tuesday heading to the inauguration, a transport official said, noting the figure was much larger than a normal work day.

The former Illinois senator was set to step up to a dais in front of the building just before noon (1700 GMT) to assume power from Bush after two terms marked by stark political division.

In the first presidential handover since the September 11 attacks of 2001, Obama was set to be sworn in by Chief Justice John Roberts.

Aides said Obama's call for all Americans to embrace public service would dominate his inaugural address, as he gets to grips with the nation's deepest recession since World War II and his plans to pull US troops out of Iraq.

Monday, Obama paid tribute to Martin Luther King, on the holiday honoring the civil rights icon's birth.

"We will come together as one people on the same mall where Dr. King's dream echoes still," Obama said.

"As we do, we recognize that here in America, our destinies are inextricably linked," he said.

"We resolve that as we walk, we must walk together. And as we go forward in the work of renewing the promise of this nation, let's remember King's lesson -- that our separate dreams are really one."

Following the inauguration of Obama and vice president-elect Joseph Biden, 66, the new US leaders were to lunch with members of Congress, Supreme Court justices and Obama's cabinet.

Marching bands, military veterans, union workers and schoolchildren were to then join a parade down Pennsylvania Avenue to the White House for Obama to take up the reins of power in the Oval Office and his place in history.

Outgoing Vice President Dick Cheney pulled a muscle in his back Monday -- the latest in a series of health problems -- while moving boxes and will be in a wheelchair for the inauguration, the White House said.

The whirlwind day was to end with 10 official inaugural balls before the Obamas could retire with their daughters Malia and Sasha, becoming the youngest First Family since John F. Kennedy's in the early 1960s.