WASHINGTON - Janet Yellen was sworn in Tuesday as the 78th Secretary of the US Treasury Department, becoming the first woman to hold the position.

Yellen was sworn in by US Vice President Kamala Harris outside the White House earlier in the day after the Senate confirmed her nomination by a vote of 84-15 on Monday.

After sworn in, Yellen said on Twitter that she is proud to be joining the Treasury Department. "Economics isn't just something you find in a textbook. It can be a potent tool to right past wrongs and improve people's lives. That's why so many of Treasury's 84,000 public servants joined the Department," she tweeted.

"Secretary Yellen is the first woman to lead the US Department of the Treasury in its 231-year history, and the first person to have served as Treasury Secretary, Chair of the Council of Economic Advisers, and Chair of the Federal Reserve," the Treasury Department said in a statement.

At a confirmation hearing last week, Yellen said her first task as treasury secretary will be helping Americans endure the final months of this pandemic and keeping people safe while getting them back to work.

Yellen also urged US Congress to "act big" with a new COVID-19 relief package as the economic recovery is losing momentum amid surging coronavirus cases.

"Economists don't always agree, but I think there is a consensus now: Without further action, we risk a longer, more painful recession now -- and long-term scarring of the economy later," she said.

US President Joe Biden has unveiled a $1.9-trillion COVID-19 relief proposal, which draws opposition from a growing number of congressional Republicans. It's unclear whether the Biden administration would secure enough votes for a new massive relief package.