WASHINGTON — Turning the page on years of war and recession, President Barack Obama summoned a divided nation Monday to act with “passion and dedication” to broaden equality and prosperity at home, nurture democracy around the world and combat global warming as he embarked on a second term before a vast and cheering crowd that spilled down the historic National Mall.

“America’s possibilities are limitless, for we possess all the qualities that this world without boundaries demands,” the 44th president declared in a second inaugural address that broke new ground by assigning gay rights a prominent place in the wider struggle for equality for all.

In a unity plea to politicians and the nation at large, he called for “collective action” to confront challenges and said, “Progress does not compel us to settle centuries-long debates about the role of government for all time — but it does require us to act in our time.”

Elected four years ago as America’s first black president, Obama spoke from specially constructed flag-bedecked stands outside the Capitol after reciting the oath of office that all presidents have uttered since the nation’s founding.

The events highlighted a day replete with all the fanfare that a security-minded capital could muster — from white-gloved Marine trumpeters who heralded the arrival of dignitaries on the inaugural stands to the mid-winter orange flowers that graced the tables at a traditional lunch with lawmakers inside the Capitol.

The weather was relatively warm, in the mid-40s, and while the crowd was not as large as on Inauguration Day four years ago, it was estimated at up to 1 million.

Big enough that he turned around as he was leaving the inaugural stands to savor the view one final time.

“I’m not going to see this again,” said the man whose political career has been meteoric — from the Illinois Legislature to the U.S. Senate and the White House before marking his 48th birthday.

In his brief, 18-minute speech, Obama did not dwell on the most pressing challenges of the past four years.

He barely mentioned the struggle to reduce the federal deficit, a fight that has occupied much of his and Congress’ time and promises the same in months to come.

He spoke up for the poor — “Our country cannot succeed when a shrinking few do very well and a growing many barely make it” — and for those on the next-higher rung — “We believe that America’s prosperity must rest upon the broad shoulders of a rising middle class.” The second reference echoed his calls from the presidential campaign that catapulted him to re-election

But his speech was less a list of legislative proposals than a plea for tackling challenges.

“We must act, knowing that our work will be imperfect,” he said, and today’s “victories will only be partial.”