WASHINGTON (AP) — New Secretary of State John Kerry reported for duty Monday, acknowledging that as Hillary Rodham Clinton’s successor he has ‘‘big heels to fill’’ and promising to protect U.S. foreign service workers from terrorist attacks overseas.

On his first day at the office in his new job, the former senator and 2004 Democratic presidential candidate was greeted with loud cheers by employees of the State Department and the U.S. Agency for International Development.