WASHINGTON - It's official.

Kirsten Gillibrand is the newest and youngest U.S. senator.

“Welcome senator,” said Vice President Joseph Biden, concluding a roughly two-minute swearing in ceremony on Tuesday.

“It's certainly something I could never have imagined. And I'm extremely grateful,” Gillibrand told reporters just after the ceremony.

Gov. David Paterson appointed Gillibrand, a Democrat from Columbia County, to fill the Senate seat vacated when Hillary Clinton was appointed U.S. secretary of state.

Gillibrand moved to the other side of Capitol Hill on Tuesday, in distance, it was just a few blocks long. But in the political realm, it was a big climb. Gillibrand started the day at her former office in the Cannon House Office Building.

A CNN news crew and a New York Daily News photographer staked out her office for more than an hour, waiting for Gillibrand to arrive, occasionally taking photographs of movers taking boxes and paintings to Gillibrand's temporary office in the Dirksen Senate Office Building.

Inside the office, aides answered the phone, “Office of the 20th Congressional District of New York,” instead of the usual greeting, “U.S. Representative Kirsten Gillibrand's Office.”

Gillibrand has become somewhat of an instant celebrity since she was selected last week to fill the Senate seat.

Yet her face is still unfamiliar to the Washington press corps.

Several times, videographers and photographers mistakenly believed that other blonde-haired women in their early 40s were Gillibrand.

Gillibrand spoke with reporters about her views on the economy and alternative energy.

One reporter asked how she balances being a member of Congress with motherhood.

Gillibrand, who is 42, has two sons, Theodore, 5, and Henry, an infant.

“It's the same way every working parent in our district does it,” she said.

Her chief of staff, at that point, told Gillibrand it was time to walk to the Capitol to be sworn in.

“OK, I'll see you guys later. I'll be around all day,” she told reporters.

Several reporters followed her down the hall. Gillibrand continued answering questions along the way.

After her swearing in, about two dozen reporters swarmed around her in the hallway outside the Senate chamber.

Gillibrand told reporters she will work diligently to convince New Yorkers that she can represent the entire state.

“Certainly, I'm going to be listening to the goals of the people I represent,” she said.

Gillibrand must run in 2010 to keep her Senate seat.

She already is taking criticism that she is not the best person for the job.

A Hispanic coalition of state lawmakers staged a demonstration in Albany on Tuesday. Gun control advocates have also been upset with Gillibrand, who has an A-rating from the National Rifle Association.

Gillibrand said she will focus on concerns New York City and upstate residents have in common.

When she was in Queens over the weekend, for example, people talked about aging sewer infrastructure, an issue many upstate communities also face, she said.

Gillibrand previously represented the 20th Congressional District, which stretches from Dutchess County north to Lake Placid in Essex County, for a little more than two years.

She said New York City residents, and those she represented in her district, share a common interest in protecting the Hudson River watershed. On the other hand, she said she will have to learn more about topics such as urban neighborhood violence, a problem that is not so common in the mostly rural 20th Congressional District.

Gillibrand said she will host community forums and meetings with elected officials across the state, just as she did in the congressional district.

“When you talk to someone who is elected locally, they know every issue and what it is all about,” she said.

Gillibrand said she expects the first measure she champions in the Senate will be a bill to prevent states from charging a $25 service fee on child support payments.

She said she spoke with Senate Majority Leader Harry Reid, D-Nevada, about it on Sunday. He suggested it would be more practical to address in a standalone bill than to address it as part of an economic stimulus package.

Gillibrand said the economy and alternative energy will be among her priorities. Rebuilding infrastructure - roads, bridges and water and sewer systems - is important, she said.

Gillibrand said she recently saw an electric car that sells for $100,000.

There needs to be a way to bring that price down to around $25,000, she said.

“These are the kinds of stimulus projects that we need because we don't want to have a 10-year recession,” she said.