President Obama described his progressive vision for the next four years in his second inaugural address on Monday, using language from the Declaration of Independence and evoking the Rev. Martin Luther King Jr. to urge Americans to embrace new measures on climate change, gay marriage, immigration reform and other government programs.

"My fellow Americans, we are made for this moment, and we will seize it, as long as we seize it together," Obama told hundreds of thousands of supporters on the National Mall in a 15-minute speech. "We must act; we must act knowing that our work will be imperfect."

Fresh off a bruising four years in office, Obama made clear that he plans to govern from the liberal end of the American political spectrum and talked at length about equality for all, including women, minorities and gay people.

"Our journey is not complete," Obama said, until America addresses a host of issues ranging from gender pay equity to gay rights to more stringent gun control laws. And he spoke in populist terms about an America that cannot succeed "when a shrinking few do very well and a growing many barely make it."

Obama mostly spoke in broad strokes, saving policy specifics for his State of the Union address next month. Missing from the president's address were specifics about solving America's growing debt crisis or persistently high unemployment. And there was little conciliatory language about bridging the nation's corrosive political divide. Instead, while couching his remarks in phrases drawn from Thomas Jefferson's 1776 declaration of separation from England, Obama pushed for initiatives on topics like climate change -- a polarizing issue he steered clear of in his first term.

"We will respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations," he said to enthusiastic applause. He also subtly echoed a remark that Republicans seized on during the campaign about no one building a business alone, saying, "no single person" can accomplish the things America needs to achieve. "Preserving our individual freedoms ultimately requires collective action," he said.

The activist tone was welcomed by some in the audience. "Amen," said Chris Sanchez, who made the trip from New York City, despite some disappointment with the president's first-term performance. "I've been waiting for him to talk about global warming for four years."

Experts said what the speech lacked in soaring rhetoric, an innovative vision or lasting phrases, it made up for in its blunt statement of intent.

"It was almost a modified campaign speech; it's the same thing Ronald Reagan did in 1981, when presenting the conservative philosophy," said Stephen Hess, a former adviser to Presidents Ford and Carter. "It wasn't an elegant speech in the way that Reagan's was, but it was channeling a liberal agenda."

Throngs of supporters gathered in the wee hours of the morning in anticipation of the nation's first black president kicking off his second term. Obama was officially sworn into office during a private ceremony at the White House on Sunday, meeting the constitutional mandate to begin his second term on Jan. 20.

Set against the backdrop of Monday's holiday for Dr. King, Obama took the oath of office with his hand on two bibles, one owned by the civil rights icon and another by President Lincoln.

Before setting out for a luncheon with congressional leaders and a stroll along part of the inaugural parade route, Obama paused on his way back into the Capitol to look out on a National Mall packed with supporters.

"I want to take a look one more time," he said. "I'm not going to see this again."