Iran will refuse to honor billions of dollars of foreign debts, Finance Minister Abol Hassan Bani-Sadr said today, calling them "debts contracted with plunderers" during the rule of the deposed shah.

The sudden move to cancel the debts which Bani-Sadr estimated at about $15 billion but which some Western sources placed as low as $7 billion, appears certain to damage the Iranian revolutionary government's already minimal credit ratings on international money markets.

Meanwhile, Rep. George Hansen (R-Idaho), here on a private mission to arrange the release of 49 hostages at the U.S. Embassy, claimed a "breakthrogh." He reported winning Iranian support for his suggested U.S. "congressional inquiry into ousted Shah Mohammad Reza Pahlavi's alleged crimes.

Hansen said tonight that Bani-Sadr had approved a plan for a congressional inquiry as a way of scaling down the hard-line demands for the shah's immediate return as a price for releasing the hostages.

Bani-Sadr, who also is acting foreign minister, presented Hansen's plan to members of the Revolutionary Council and won their agreement, too. Hansen told reporters tonight. But some analysts doubted whether the radical Islamic students holding 49 American hostages would trust a purely American investigation, and there was no indication of the postion of Iran's de facto ruler, Ayatollah Ruhollah Khomeini, on the proposal.

Hansen was allowed to visit U.S. EMBASSY charge d'Affaires Bruce Laingen and two others detained in the Foreign Ministry.

Through Bani-Sadr's intervention, Hansen said he had briefly visited Laingen, political officer Victor Tomseth and Col. Leland Holland, the embassy security officer, Thursday night.

Laingen, he said was "well but concerned and tired."

In his economic message, Bani-Sadr first indicated that all foreign debts were to be canceled. But he later said he meant only those involving loans from 28 private banks -- most of them joint ventures between Iranians and foreign interests -- that were nationalized in July.

On television tonight, Bani-Sadr suggested that the debts were contracted by bank managers who suspected the nationalization was imminent and who then diverted funds to their own personal accounts.

"How can we accept these debts contracted with plunderers?" he asked, "we are not going to pay them."

Six of the 28 banks nationalized have U.S. participation, and the American banks are the largest foreign financial institutions working here.

By far the largest American bank was the Chase Manhattan, run by David Rockefeller. He has incurred the Iranian revolutionaries' special animosity for his crucial role in paving the entry into the United States of ousted Shah Mohammad Reza Pahlavi.

Other American banks were Continential Illionis, Citibank of America, First Boston Corp. and the Mellon Bank.

A central bank offical said the Chase Manhattan was especially suspect because it handled most of the shah's personal fortune and enjoyed a privleged position as the conduit for all oil transactions under the former government and unfairly caused a $500 million loan to go into default by "losing" an Iranian interst payment last month.

Bani-Sadr's repudiation of these debts, long one of his favorite goals, was related to a series of decisions designed to punish American intersts in Iran.

Starting last week, Bani-Sadr ordered the withdrawal of Iranian deposits in American banks and refused to accept dollars in payment for Iranian oil.

The United States froze the Iranian governments's assets in the United States and tried to do the same with branches of U.S. banks abroad.

As result of the decision today, however, could turn out to penalize the European banks since their governments have not frozen Iranian assets.

Hansen's proposal came after a day in which a dozen or so Americans living in Iran joined thousands of militant Iranians in the daily pro-Khomeini march in the streets and endorsed Iran's call for the shah's extradition.

"Having lived here through the revolution, we know him to be a murderer," a statement distributed by several American and British residents of Tehran said. They could not be immediately identified.

Hansen, who apparently is operating privately with no known official role, said, "I have confidence that when Congress opens an inquiry into the charges against the former shah, it will be most useful here and in the United States. It should ease up the situation for the hostages."

He insisted that "no deals" were involved, but rather, "the idea that a congressional investigation might be a helpful step."

"Americans do not appreciate coverups or kangaroo courts," he said, and "congressional inquiries could reflect the real American morality and fair-mindedness that Iranians and all people have grown to expect."

Hansen and former Irish foreign minister Sean MacBride, an official of the human rights group Amnesty International who is here at the Iranian government's invitation, have been authorized to visit the American hostages at the embassy Saturday, according to informed sources.

At face value, diplomats agreed with Hansen's evaluation of his visit, which he had credited with opening up a dialogue in the deadlocked crisis so far marked by "insensitive and provocative acts by both sides fueled by public posturing of the media and the absence of any meaningful official contacts between the governments."

The diplomats saw the virtue in the fact that a congressional inquiry required no formal approval from the White House, and thus could spare President Carter the embarrassment of making a formal gesture of apology to open up dialogue between the two governments.

However, some analysts doubted whether the radical Islamic students at the embassy would trust a purely American investigation.

MacBride, the 75-year-old Nobel and Lenin peace prize winner, told newsmen today that he felt "it was up to the United States to make a gesture."

MacBride, who saw the foreign minister twice, suggested the United States make a voluntary declaration to the effect that it "wanted to establish close and friendly relations with Iran and not in the future use the embassy as a center for subversive activities. CAPTION: Picture, ABOL HASSAN BANI-SADR . . . calls banks "plunderers"