President-elect Barack Obama speaks at the Lincoln Memorial during an inaugural concert in Washington on Sunday. (Charles Dharapak, Associated Press)

The United States of America will reach a political milestone on Tuesday, when Barack Obama, standing on the steps of the U.S. Capitol, is sworn in as the country's first black president.

With his right hand in the air and his left hand resting on Abraham Lincoln's inaugural Bible, Obama will take the oath of office at noon ET.

As many as four million people, the largest crowd in inaugural history, are expected to throng to the National Mall, where giant telescreens have been scattered throughout the area, so spectators can watch Obama become the 44th president of the United States.

Even before the sun rose Tuesday, crowds of people had begun working their way to the Mall, a grassy expanse of parkland linking the Capitol to the Lincoln Memorial.

The 47-year-old former Illinois senator will be sworn in by Supreme Court Chief Justice John Roberts. Michelle Obama, the wife of the president-elect, will carefully hold Lincoln’s Bible, which hasn’t been used since he took the oath of office in 1861.

Obama, who enters the Oval Office facing significant domestic and foreign policy challenges, including a deep economic crisis and wars in Afghanistan and Iraq, is expected to call for a new "era of responsibility" when he delivers his highly anticipated inaugural speech.

The address, to last about 20 minutes, is expected to ask Americans to reject the philosophy of "me first" and call on individuals, corporations and businesses to take responsibility for their actions, CNN reported.

Obama has been studying the inaugural speeches of Lincoln and Franklin D. Roosevelt to prepare himself for the event.

Tight security

On Monday, Martin Luther King’s national holiday, Obama appealed to Americans to honour King through service to others.

"I ask the American people to turn today's efforts into an ongoing commitment to enriching the lives of others in their communities, their cities, and their country," Obama said.

Security in Washington is unprecedented, with more than 25,000 law enforcement officials scanning and scouring the area, including sharpshooters, air patrols, bomb-sniffing dogs, U.S. Coast Guard patrol boats along the Potomac River, and more than 5,000 security cameras.

Defence Secretary Robert Gates has been chosen to be the "designated successor" in case of an attack at the ceremony. In the past, presidents have designated one cabinet member or other official to take the reins of government if there is a calamity at the inauguration. Gates will be kept at an undisclosed location during the event.

An hour before the inauguration, President George W. Bush, the president-elect and their families will leave the White House by limousine. It is expected that Bush will follow the tradition of past presidents and leave a personal letter for the incoming commander-in-chief.

Following the swearing-in ceremony, Bush will speak at a departure ceremony at Andrews Air Force Base before boarding a plane that will fly him to Midland, Texas. Later, he will travel to his ranch at Crawford.

Around 2 p.m., after the signing ceremony in the president’s room in the Capitol, an inaugural luncheon and the review of troops on East Front, Obama and new vice-president Joe Biden will lead the inaugural parade. Followed by more than 10,000 people representing various school bands and military honour guards, they will travel from Capitol Hill along Pennsylvania Avenue to the White House.

Obama will then spend the evening attending 10 official inaugural balls.

In Lincoln's footsteps

Celebrations for the next presidency began on the weekend, with Obama's whistle-stop tour, from Philadelphia to Washington, along the path Abraham Lincoln took in 1861. That followed a celebrity-filled concert where several hundred thousand people flanked the reflecting pool, hearing actors, singers and then Obama himself rally for national renewal.

On Monday evening, Obama was attending three private dinners to honour former secretary of state Colin Powell; Biden, a longtime senator from Delaware, and Senator John McCain, the 2008 Republican presidential nominee.

Michelle Obama was hosting a children's evening concert.

On the eve of the inauguration, Prime Minister Stephen Harper wished the president-elect well, but added that "nobody is under any illusion" about the challenges he faces, including the economic meltdown.

"If I were in his position, on the economic side, there would be two [priorities] high on my list. One is the stimulus package and getting that right," he said.

Harper also urged Obama to continue trying to stabilize the financial sector in the United States.

"This will remain a major challenge to the world economy until it is fixed," he added.

With files from the Associated Press and the Canadian Press