The Iranian Embassy and its diplomatic staff here were seized today by three armed men who described themselves as Arab nationalists from the oil-rich Khuzestan Province in southwestern Iran.

They threatened to kill about 20 hostages and blow up the embassy by noon Thursday (7 a.m. EDT) unless 91 Arab dissidents imprisoned by the Iranian government in Khuzestan are released and flown to London. The gunmen then want another plane to fly them and the released prisoners out of Britain.

Most of the hostages are Iranian diplomats, apparently including the charge d'affaires here, G. Ali Afrooz. Three journalists, two British and one Lebanese, also are being held along with the armed British police officer guarding the embassy when the gunmen burst in shortly after noon today.

An unidentified Iranian woman who worked in the embassy was released unconscious by the gunmen late this afternoon after she apparently collapsed from the strain. Police and the gunmen also were negotiating about getting a physician into the embassy to treat a "badly injured" male Iranian diplomat who may have been shot when the embassy was invaded.

The gunmen told police and reporters outside the embassy that they were "Arabs from Arabistan" -- their name for Khuzestan -- and were fighting for autonomy for the province that contains most of Iran's oil and its refineries and shipping facilities.

Khuzestan also is home to one of Iran's several large non-Persian minorities, more than a million Arabs. They have been backed by neighboring Iraq in periodic sabotage of oil-producing facilities and uprisings against the Iranian central government in Tehran both before and after the Islamic revolution of the Ayatollah Khomeini.

In Tehran, the Iranian Foreign Ministry blamed Iraqis for the takeover. A spokesman said Iran had expected such an action because Iraqis attempted the same thing a few days ago in Lebanon.

[Neither the spokesman nor Iranian television news tonight mentioned reports that the embassy captors were dissident Iranian Arabs from the oil-producing province of Khuzestan, Washington Post correspondent William Branigin reported from Tehran.]

Before Iran and Iraq reached a compromise to end their propaganda warfare in 1975, Iraq armed and financed an Ahwaz Liberation Front to free Khuzestan. Since their relations deteriorated again following the Islamic revolution in Iran, Iraqi officials have made no secret that they were helping Khuzestan's Arab population by all possible means.

Hundreds of Arab militants were killed in uprisings against Iranian soldiers in Khuzestan last year, and hundreds were arrested only last week after riots in the Khuzestan capital of Ahwaz. Most have since been released.

The three gunmen who seized the Iranian Embassy here have made clear in contacts with police and reporters and in messages sent out by the captive journalists that they are seeking both the release of the 91 remaining prisoners and worldwide publicity for their cause.

"We want the freeing of 91 prisoners and the recognition of our region as an autonomy," the unidentified leader of the gunmen said in a telephone interview tonight with a BBC reporter calling from outside the embassy. "We want to send our voice to the world. We are fighting for justice and the autonomy of Arabistan."

Asked if the gunmen and their compatriots were rebelling against Khomeini, he said, "We are not against Ayatollah Khomeini. We are against every leader who does not want to give us our legal rights."

The gunmen are negotiating with British police officers and interpreters in both English and Arabic on the telephone and in shouted conversations through an open third-floor window and the shut front door of the embassy. It is one of a row of about 20 white-washed, Georgian townhouses with pillared porticos on a small picture-postcard street facing spacious, green Hyde Park in a wealthy international neighborhood in the middle of London.

At a news conference tonight in the restaurant of the Royal Albert Hall, a block from the embassy, Scotland Yard Deputy Assistant Commissioner John Dellow said police told the gunmen, "We want to continue talking and want to resolve this situation without the loss of life." He said the "delicate negotiations" were "going calmly" and would be handled entirely by the police, although they were keeping the British government informed.

Soon after the embassy was seized, the British Foreign Office informed the Iranian government of what was happening, making clear no Britons were responsible, and promised to take all possible steps to end the incident quickly and peacefully. Because the embassy remains Iranian territory under international law, officials here said, British authorities could not enter without permission from Tehran or Iranian diplomats here.

Among the people inside the embassy when the gunmen invaded, pushing the police guard in ahead of them at gunpoint, were three newsmen. Lebanese journalist Mustafa Karkouti was interviewing someone inside, and BBC television newsman Chris Cramer and news film cameraman Sid Harris were applying for visas to Iran.

Karkouti first transmitted the gunmen's demands under their orders in a telephone call to senior duty editor Mike Brown of BBC Radio's World Service here. "The gunmen have asked for 91 Arab prisoners in what they described as an Arab land to be released from jail," Brown told reporters later. "They want them to be flown to London, and then to be provided with a plane to fly out of London."

In a later call to the BBC World Service, Karkouti relayed the gunmen's threat to kill hostages and blow up the embassy if their demands were not met by noon Thursday.

Tonight, again under orders from the leader of the gunmen, Cramer sent telex messages to the BBC television newsroom from a telex machine inside the embassy. They briefly described the storming of the embassy, relayed the gunmen's demands and assurances of the hostages' safety, and conveyed apologies to the British people and government from "The Group of the Martyr."

Still later, the leader of the gunmen spoke on the telephone to a reporter at BBC in a conversation that was taped and broadcast on television and radio.

As armed police wearing flak jackets settled in for a long wait under the glare of television and searchlights late tonight, other officers had to be hurried to the scene to hold back dozens of chanting Iranian students from London who said they wanted to go into the embassy and take the places of the hostages.

"It is unfair that these people should be held in there," said a spokesman for the Iranian students, Jafar Chitsaz. "This is an Iranian crisis. It should be solved by Iranian people."

Iranian Foreign Minister Sadegh Ghotbzadeh was quoted tonight as saying during a visit in the United Arab Emirates that "Iran will not yield to the demands of individuals, governments or groups if this is incompatible."

Ghotbzadeh said that Iran would execute prisoners being held in jail there if any Iranian Embassy staff members were harmed.