President Barack Obama said America "must act" on several pressing issues if it wants to live up to the promise of its Founding Fathers during his second inaugural address Monday.

Obama made a case for his vision of America's future while invoking past presidents and civil rights leaders.

"We have always understood that when times change, so must we; that fidelity to our founding principles requires new responses to new challenges; that preserving our individual freedoms ultimately requires collective action," Obama said in front of a crowd of roughly 600,000 on the National Mall on Martin Luther King Jr. Day.

The president outlined several key issues he plans to tackle in his second term, including taking on the gap between the rich and the poor, overhauling immigration and gun control laws and addressing climate change.

"My fellow Americans, we are made for this moment, and we will seize it, so long as we seize it together," Obama said.

Obama, who won re-election in November largely on a theme of economic fairness, said true economic recovery cannot occur if the income gap between the rich and the poor remains large and if the middle class does not grow.

"We, the people, understand that our country cannot succeed when a shrinking few do very well and a growing many barely make it," Obama said. "We believe that America’s prosperity must rest upon the broad shoulders of a rising middle class."

He defended entitlement programs such as Social Security and Medicare while rejecting the notion that reducing the federal deficit comes down to stripping the social safety net.

"We reject the belief that America must choose between caring for the generation that built this country and investing in the generation that will build its future," he said.

After dodging the issue publicly for most of his first term, he promised to make America a leader in reversing climate change by investing in renewable energy.

"Some may still deny the overwhelming judgment of science, but none can avoid the devastating impact of raging fires and crippling drought and more powerful storms," Obama said.

He spent a part of his speech addressing the United States' role overseas.

"Enduring security and lasting peace do not require perpetual war," the president said.

Obama said that while "a decade of war is now ending," America must continue to support democracies worldwide.

"We must be a source of hope to the poor, the sick, the marginalized, the victims of prejudice," he said.

The president, who was sworn in using two Bibles, one that belonged to King and another that belonged to President Abraham Lincoln, equated the gay rights movement with the Civil Rights Movement King led in the 1960s.

"Our journey is not complete until our gay brothers and sisters are treated like anyone else under the law, for if we are truly created equal, then surely the love we commit to one another must be equal as well," he said.

Obama became the first president to endorse gay marriage last year.

He also promised to take on immigration reform, which evaded him in his first term, and renewed his pledge to reform gun laws.

"Our journey is not complete until all our children, from the streets of Detroit to the hills of Appalachia to the quiet lanes of Newtown, know that they are cared for and cherished and always safe from harm," he said.

Obama unveiled a series of gun control proposals last week, including a ban on assault weapons and stricter background checks.

Obama urged Republicans and Democrats to set aside their differences to address the country's needs and decried the often partisan nature of political discourse.

"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," he said. "...We cannot mistake absolutism for principle or substitute spectacle for politics or treat name-calling as reasoned debate."

The address was short on specifics as most inaugural addresses are but was heavy on historical references, mostly to the Founding Fathers, the nation's founding and the Civil Rights Movement.

The oaths Obama and Vice President Joe Biden took Monday were ceremonial. Because Jan. 20 fell on a Sunday this year, Obama and Biden were officially sworn-in during a quiet White House ceremony Sunday. It is tradition to not hold an inauguration ceremony on a Sunday.

Before his speech, Obama and First Lady Michelle Obama had coffee and tea with congressional leaders in the White House, perhaps trying to reset his often rocky relationship with congressional leaders in the Republican Party.

Former Presidents Bill Clinton and Jimmy Carter were both in attendance. Former Presidents George W. Bush and George H. W. Bush did not attend as the elder Bush continues to recover from recent health problems.

Singers Beyonce, James Taylor and Kelly Clarkson performed during the inauguration ceremony.

Soon after his speech, Obama officially nominated his picks for Secretary of State (Sen. John Kerry, D-Mass.), Secretary of Defense (former Sen. Chuck Hagel, R-Neb.), Treasury Secretary (Chief of Staff Jack Lew) and CIA Director (White House security advisor John Brennan).