Hillary Rodham Clinton has formally resigned as the US secretary of state. Former Massachusetts Senator John Kerry has replaced Clinton after taking the oath of office.
Clinton bid goodbye Friday to her staff at the State Department in Washington.
"I know the world we are trying to help bring in to being in the 21st century will have more difficult days," she said. "But I am more optimistic standing here today than I was four years ago."
Her resignation became effective after the swearing-in of her successor, John Kerry, who took the oath of office in a private ceremony on Friday. Kerry previously served as a senator of Massachusetts and ran as the Democratic candidate in the 2004 presidential election.
Clinton sent her letter of resignation to President Barack Obama on Friday shortly before she was to leave the State Department for the last time in her official capacity.
In her letter, Clinton thanked the president for the opportunity to serve in his administration and said it had been an honor to be part of his Cabinet.
"I am more convinced than ever in the strength and staying power of America's global leadership and our capacity to be a force for good in the world," she wrote in the letter.
Despite criticism for her administration's handling of the September attack on the US diplomatic mission in Benghazi, Libya, that killed four Americans, Clinton is leaving office as one of American politic's most popular figures.
Clinton’s last day as secretary of state came on the same day that a suicide bomber attacked the US Embassy in Ankara, killing himself and one Turkish national security guard.
"We live in a very complex ... and very dangerous time as we saw just today at our embassy in Ankara," Clinton said in her farewell speech, noting she had spoken to her Turkish counterpart and the US ambassador there.
During her term as secretary of state, Clinton traveled to 112 countries, a record totaling 401 travel days in a four-year span.
hc/kms (AP, dpa)