The shah's portrait, which in the last month has been up, down and then up again at the Iranian embassy here, was down once more yesterday as rejoicing supporters of Ayatollah Ruhollah Khomeini seized control of the building in another bloodless mini-coup.

Led by 12 pro-Khomeini Iranian diplomats, the group tore down and destroyed dozens of pictures of Shah Mohammad Reza Pahlavi, replacing them with equally imposing images of the ayatollah.

They hack-sawed their way into the office of longtime Iranian Ambassador Ardeshir Zahedi, and there staged a press conference where they proclaimed themselves the representatives of the new Iranian government and installed one of their own -- diplomat Jafar Fagih -- as the interim replacement for Zahedi.

Outside the embassy and its hallways, Iranian music blared over a shortwave radio. A crowd of about 200 Iranian diplomats, students, businessmen, and children hugged and kissed one another as banners proclaiming "victory to the revolution" were hoisted above them.

This takeover was designed to be more permanent than the ones that preceded it. Shahriar Rouhani, a 29-year-old Yale graduate who represents the ayatollah, announced that a permanent ambassador would soon be appointed and that his government hoped for friendly diplomatic relations with the United States following "immediate recognition" of Khomeini's authority.

The takeover was apparently also much more carefully planned than the ones in the past.

Fagih said he telephoned the military attache -- who had stationed armed guards at the embassy after the first takeover in January -- and asked that they be removed. They were.

The first force of three or four proayatollah Iranians who arrived at the embassy entered by knocking on the door. They were admitted by the charge d'affaires, according to witnesses, who greeted them with an embrace.

Once inside the building, the Iranians used a hack-saw blade to cut the lock on the office of Zahedi, who was said to be out of the country. One of the first places they headed for were the file cabinets, which turned out to be empty.

Rouhani charged that among the files removed from the embassy were documents pertaining to "payoffs to American dignitaries," including members of Congress. He said he would not disclose details of the "payoffs," except to note that "they were indirect" and "not just caviar and champagne."

Zahedi had a "caviar list, a champagne list and a silk-rug list," agreed a ranking embassy source, but "God knows some other things were given away."

Fagih said Zahedi, whose largess was celebrated in Washington, had a staff of four or five persons just to wrap and send various gifts.

An embassy spokesman has said previously that the only things Zahedi took with him when he left Washington were personal effects, such as clothing.

Yesterday, Rouhani and Fagih took pains to stress their hopes for continued friendly relations with the United States.

Though they removed a portrait of Richard Helms, the former ambassador to Iran and once head of the CIA, they were careful not to damage a large plastic peanut in Zahedi's office, apparently a symbol of Jimmy Carter's presidency.

"We believe there are genuine grounds for friendship between the American people and the Iranian people," Rouhani told his first embassy press conference. "The oil will flow so long as there is a friendly relationship."