Hillary Clinton formally resigned as the 67th secretary of state, just moments ago.

In a letter to President Obama, she said it was an "honor to serve."

"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.

She concluded, "On a personal note, it has been a pleasure to work with you and your team. Thank you, Mr. President, for your friendship, and for the opportunity to serve in your Cabinet."

According to the AP, her resignation is effective as soon as former Sen. John Kerry is sworn in as the 68th secretary of state. That's expected to happen in a private ceremony later today.

Update at 2:41 p.m. ET. A Farewell:

Just minutes ago, Secretary Clinton said farewell to State Department employees.

"I'm proud to have been a part of you," she said during an address at the State Department.

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