Israeli authorities on Monday expelled Richard Falk, a United Nations investigator of human rights in the Palestinian territories, saying he was unwelcome because of what the government has regarded as his hostile position toward Israel.

Mr. Falk, an American, arrived in Israel on Sunday. He was held at the airport and placed on the first available flight back to Geneva, his point of departure. A spokesman for the Israeli Foreign Ministry said that Mr. Falk had been informed in advance that his entry would be barred. Mr. Falk was not immediately available for comment.

Mr. Falk, a professor of international law at Princeton, has the title of United Nations Human Rights Council special rapporteur for the Palestinian territories. He has long been criticized in Israel for what many Israelis say are unfair and unpalatable views.

He has compared Israel’s treatment of the Palestinians to Nazi atrocities and has called for more serious examination of the conspiracy theories surrounding the Sept. 11 attacks. Pointing to discrepancies between the official version of events and other versions, he recently wrote that “only willful ignorance can maintain that the 9/11 narrative should be treated as a closed book.”

In his capacity as a United Nations investigator, Mr. Falk issued a statement this month describing Israel’s embargo on Gaza, which is controlled by Hamas, as a crime against humanity, while making only cursory reference to Hamas’s rocket attacks against Israeli civilian centers. Israeli officials expressed outrage.

When his appointment was announced by the Human Rights Council last spring, the Israeli representative said it was “impossible to believe that out of a list of 184 potential candidates,” the members had made “the best possible choice for the post.”

The American and Canadian representatives also expressed concerns about Mr. Falk’s possible bias. The Palestinian representative said it was curious that Israel was “campaigning against a Jewish professor” and called the nomination “a victory for good sense and human rights.” Israel objects to the mandate of the special rapporteur on grounds that it ignores all human rights violations by Palestinians, either against Israelis or against other Palestinians. More specifically, it objects to Mr. Falk.

A statement issued on Monday by the Foreign Ministry noted that in the past three years, Israel welcomed visits by seven special rapporteurs of the Human Rights Council and two other senior United Nations representatives.

In Mr. Falk’s case, it continued, his “vehement publications” made it “hard to square his appointment” with the council’s own requirements, which call for envoys to be impartial and objective. The council’s own procedures require its envoys to operate with the consent of the state concerned.

A Foreign Ministry spokesman, Yigal Palmor, said that Mr. Falk had come to Israel in June for what was supposed to be a personal visit, but had instead carried out work as a rapporteur. “He lied,” Mr. Palmor said.

Regardless of Mr. Falk’s views, some Israelis questioned the wisdom of banning him, noting that it would hardly make his reports more sympathetic.

Jessica Montell, the executive director of B’Tselem, an Israeli group that monitors human rights in the occupied territories, said that even if Israel had “legitimate concerns about Professor Falk’s mandate,” barring his entry was “an act unbefitting of democracy.”

Also on Monday, Israel released 224 Palestinian security prisoners from its jails as a gesture to the Palestinian Authority president, Mahmoud Abbas.

Most of those released were serving sentences of five years or less. None had been convicted of deadly attacks on Israelis, and none were from Islamic groups hostile to the Palestinian Authority, like Hamas.

Israel has released almost 1,000 Palestinian prisoners in the past 18 months in an effort to strengthen the Western-backed administration of Mr. Abbas. At least 9,000 remain in Israeli jails.