CHICAGO — Rahm Emanuel, the political pitbull who served as President Barack Obama's first chief of staff, was sworn in Monday as mayor of Chicago, pledging a "new day" for a US city facing a host of financial, safety and educational struggles.

Emanuel took over from Richard Daley, who had served as city chief executive for the past 22 years. When Daley surprisingly decided not to seek another term last year, Emanuel left the White House for Chicago, where he was elected with 55 percent of the vote in February after overcoming a residency challenge.

"New times demand new answers; old problems cry out for better results," Emanuel told a crowd of thousands at Chicago's Millennium Park that included Vice President Joe Biden.

"This morning, we leave behind the old ways and old divisions and begin a new day for Chicago," Emanuel said. "I am proud to lead a city united in common purpose and driven by a common thirst for change."

The city's first Jewish mayor took the oath standing next to his wife Amy and their three children, whom he immediately kissed and hugged after being sworn in.

Emanuel, 51, served in president Bill Clinton's White House in the 1990s, then went into the private sector and made millions before being elected to the US Congress in 2002 from Chicago's northwest side.

He led the efforts of the Democratic Party to re-capture the House of Representatives in 2006 and was selected by Obama to serve as his chief of staff in 2008.

In his speech Emanuel thanked Obama, a native of Chicago and former US Senator from the state of Illinois, for allowing him to come back and run for mayor.

"He understood why I wanted to come home to get our city moving again," Emanuel said.

While praising Daley, the new mayor sought to differentiate himself and tacitly criticized the policies of the man he once worked for.

"From the moment I began my campaign for mayor, I have been clear about the hard truths and tough choices we face: we simply can't afford the size of city government that we had in the past," Emanuel said.

"And taxpayers deserve a more effective and efficient government than the one we have today."

Emanuel comes into office facing a huge collection of problems starting with a budget deficit close to $600 million. He was expected to put a freeze on city spending later today.

"Given the challenges we face, we need to look for better and smarter ways to meet our responsibilities. So when I ask for new policies, I guarantee, the one answer I will not tolerate is: 'We've never done it that way before.'"

And yet the shrewd politico, whose uncompromising style earned him the nickname "Rahmbo," is expected in some ways to replicate Daley's iron-fisted tactics and focus on maintaining Chicago's reputation as "the city that works."

In his February victory speech the famously foul-mouthed Emanuel gave a nod to his mayoral predecessor, saying Chicago bore Daley's "indelible imprint."