The biggest bloc of developing countries at the United Nations has chosen the observer state of Palestine as its next leader, the Palestinian ambassador said Tuesday, a decision that could cause new tensions with Israel and the United States.

The ambassador, Riyad H. Mansour, said in a telephone interview that Palestine would take over leadership of the bloc, known as the Group of 77, from Egypt, starting in January.

Originally 77 countries when formed in 1964, the bloc’s membership has nearly doubled in size. The bloc often speaks as one voice in the General Assembly and represents more than 80 percent of the world’s population.

“We will be negotiating on behalf of 135 countries,” Mr. Mansour said, a total that would include his own delegation.