Iranians holding visas to enter the United States were turned away from planes at London airports yesterday, following President Carter's latest crackdown in response to the hostage crisis.

But administration officials said the strict new controls will not affect great numbers of people at first because most Iranian visa-holders -- students, tourists and businessmen -- already are in the country and are thus protected from immediate deportation.

Carter announced Monday he was canceling all visas issued to Iranians for entry into the United States and warned that they would be revalidated only for "compelling and proven humanitarian reasons or where the national interest requires."

Since the 50 American hostages were seized Nov. 4, more than 14,000 Iranians have been admitted to the United States -- about half of them religious minorities who fear persecution under the regime of Ayatollah Ruhollah Khomeini, officials have said. About 14,700 Iranians have left the United States in the same period, according to figures from the Immigration and Naturalization Service.

State Department and immigration officials said yesterday that concern about religious or political persecution in Iran would be viewed as a valid humanitarian reason for revalidating the visas of those now being barred from the United States.

White House press secretary Jody Powell said at a briefing yesterday that the new rules "won't have an immediate impact on large numbers of people." He noted that more than 56,700 Iranian students here hold visas good for as long as they remain in school. Others whose visas expire can apply for asylum and be protected by U.S. law.

"Once in the good old United States legally, or illegally for the matter, they are cloaked in the mantle of the constitutional and legal protections we all value," Powell said.

More than 200 students have left the country since their visas were found invalid last fall. Some 2,500 more have been ordered out and 7,700 face deportation hearings, an INS spokesman said.

State Department spokesman Hodding Carter said in a separate briefing that the administration would take a close look at Iranian students trying to renew visas, with the view that "we're not interested in prolonging essentially frivolous stays in the United States."

While the administration's public posture has been a hard line against Iranian visa holders, its unstated policy has aimed at trying to help the religious minorities who have fled Iran, several officials said.

Some private and congressional refugee experts refuse to discuss the issue because of fear that Khomeini will retaliate against those religious minorities -- mostly Jews and Bahais -- who remain in Iran.

The Washington Post reported last month that it was estimated that half the 80,000 Jews in Iran had left since the revolution that toppled Shah Mohammed Reza Pahlavi. Most went to Israel, but thousands entered the United States, officials said.

Applications for political asylum by Iranians now in the United States have not been processed by the State Department since the revolution because of the unsettled political situation in Iran, a spokesman said.

Since the hostages were seized last November, about 1,000 applications have been received -- most from students facing deportation. Several hundred more already were pending. The spokeswoman for the department's office of humanitarian affairs said there would be no decision on those applications until the hostage question is settled.

Those who apply for asylum are in a kind of legal limbo, but won't be forced to leave the country, officials said.

Administration officials said they expected an increase in asylum applications because of the decision to take a hard look at visa renewals.

An estimated 150,000 Iranians hold visas for the United States. Students holding valid visas who have left the United States on vaction could have a hard time getting back in, officials predicted.

"If they were visiting a sick aunt, that would fit the humanitarian exception," one said. "If they went skiing, they could be in trouble."