WASHINGTON // John Kerry was sworn in to replace Hillary Clinton, who ended a four-year tenure as secretary of state that made her one of America's most popular public figures, despite leaving amid a feud over the attack on the US diplomatic mission in Libya.
In a letter to president Barack Obama, Mrs Clinton thanked her former rival for the 2008 Democratic presidential nomination for the opportunity to serve in his administration. Mrs Clinton said it had been an honour.
"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 said in the letter.
The former first lady, once considered a divisive figure in American politics, leaves office as one of its most popular. But she remained coy about whether she would run for president in 2016. "I am making no decisions, but I would never give that advice to someone that I wouldn't take myself," she said.
Her resignation became effective at 4pm on Friday, when supreme court justice Elena Kagan swore in Mr Kerry as the top US diplomat.
Mrs Clinton also left office with a slap at critics of the Obama administration's handling of the September attack in Libya. She said that critics of the administration's handling of the attack do not live in an "evidence-based world" and their refusal to "accept the facts" is unfortunate and regrettable.
Mrs Clinton said the attack in Benghazi was the low point of her time as America's top diplomat.
After a bomber exploded a device outside the US Embassy in Ankara, Turkey, on Friday, killing himself and a guard, Mrs Clinton said in her farewell speech the attack showed again how "we live in very complex and dangerous times".