Barack Hussein Obama was today sworn in as 44th President of the United States in front of possibly the largest mass of humanity ever to have gathered in one place for a single political moment.

As many as 2 million people in ­Washington's National Mall heard their new commander-in-chief deliver a sombre 20-minute speech in which he acknowledged that the country was in the midst of crisis – mired in wars, its economy struggling and its national confidence sapped.

He promised the largely silent crowd that the challenges would be met, but warned it would take time, some sacrifice, a new form of politics and a re-engagement with the world, in which America would recognise that "power alone cannot protect us, nor does it entitle us to do as we please".

President Obama took the oath just after midday under a crisp and ­cloudless azure sky in front of the glistening cream dome of the Capitol, which, it is now accepted, was partly built by slaves.

The day, cold enough to freeze breath, had begun with millions of individual journeys by coach, train and on foot as the crowds began converging before dawn for a moment widely taken as one of renewal and of double foreclosure. This was to be the end of the last eight years of Republican rule and of the bars which, at any previous time in history, would have made the election of an ­African American president unthinkable.

They had come to celebrate – and for days they had been doing just that in parties and balls all over town. The cheer as Obama swore his oath on ­Lincoln's Bible rippled and roared all the way from the Lincoln Memorial to the Capitol, nearly two miles away.

But when Obama spoke it was immediately apparent that the tone of this inauguration was grave, addressed as much to the hundreds of millions tuned in around the world as to the shimmering sea of upturned faces in front of him.

"That we are in the midst of crisis is now well understood," he said. "Our nation is at war, against a far-reaching network of violence and hatred. Our economy is badly weakened, a consequence of greed and irresponsibility on the part of some, but also our collective failure to make hard choices and prepare the nation for a new age."

But Obama said he was ready to meet the challenges confronting the US as he called for a return to traditional values of hard work and honesty.

"What is required of us now is a new era of responsibility – a recognition on the part of every American that we have duties to ourselves, our nation and the world ..."

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