Washington/Philadelphia  - Calling for the country to work toward a more "perfect union," Barack Obama embarked Saturday on a historic train journey, the kick-off leg of four days of celebration leading up to his inauguration as the country's first African- American president on Tuesday.

Obama spoke to his fellow travellers and supporters in Philadelphia's 30th Street train station, recalling how 18th century farmers, lawyers and merchants had gathered in the city to launch the American war for independence despite the "insurmountable odds" they faced.

"We are here today not simply to pay tribute to our first patriots but to take up the work that they began," Obama said.

The "Whistle Stop" train tour follows the last legs of president Abraham Lincoln's own inaugural trip to Washington nearly 150 years ago, and takes place under similarly tight security.

Lincoln was under death threats for his commitment to ending slavery, and had to pass through Baltimore in the dead of night in disguise to avoid attacks.

While Obama's journey takes place in bright sunlight and a deep chill - minus 12 degrees centigrade - similar security against more sophisticated threats was in place for Obama's train route.

The entire inauguration ceremony carries a Lincoln theme - "A New Birth of Freedom" - taken from Lincoln's famous Gettysburg Address that followed an historic 1863 Civil War battle.

Throngs of supporters are expected to line the 220-kilometre train route. Aside from scheduled campaign stops in Wilmington, Delaware, where vice president elect Joe Biden will board, and Baltimore, the train will make a few "slow rolls" through small towns in Delaware and Maryland to give supporters lining the tracks more time to wave.

In his broadcast departure speech from Philadelphia, Obama noted that the trials faced by the current generation were "different, but severe in their own right," compared to the founding fathers of 1776. He was referring to the faltering economy, wars in Iraq and Afghanistan and the warming planet.

But he was confident that if the country displayed the same "perseverance and idealism" as the founding fathers, the problems could be mastered.

"The American Revolution was - and remains - an ongoing struggle 'in the minds and hearts of the people' to live up to our founding creed," Obama said. "Starting now, let's take up in our own lives the work of perfecting our union."

In his radio address earlier Saturday, Obama promised to make his inauguration a transparent and inclusive occasion.

"The events of the next several days are not simply about the inauguration of an American president," Obama said. "They will be a celebration of the American people."

A free welcome music concert Sunday night will take place on Washington's National Mall in front of the Lincoln Memorial, with a star-studded caste including Beyonce and Stevie Wonder expected to draw 1 million people.

On Monday, Obama has declared a nationwide "call to service" in honour of civil rights pioneer Martin Luther King Jr's birthday, prompting events across the country that highlight volunteerism.

On Tuesday, about 2 million people are expected to crowd the National Mall for the big moment. Obama is to be sworn in at noon on the steps of the US Capitol, placing his hand on the same Bible that Lincoln used at his own swearing in on March 4, 1861

In an unusual step, Obama's inauguration committee said it would open the White House on Wednesday, his first full day at home in his new quarters, to "first come, first served" people who responded to the e-mail.

For the first time, there will also be an official "neighbourhood ball" with emphasis on attendance by ordinary Washingtonians. (dpa)