The three gunmen holding about 20 hostages in the Iranian Embassy here allowed at least two deadlines to pass without violence today as police established a pattern of calm drawn-out negotiations to try to end the embassy siege peacefully.

After seizing the building late yesterday morning, the gunmen had threatened to kill all of the Iranian diplomats among their hostages and blow up the embassy by noon today if the Iranian government did not free 91 fellow Arab minority militants imprisoned in the oil-rich province of Khuzestan in southwestern Iran.

After the deadline, they demanded that Iran indicate two hours later that it was involved in the negotiations. Although the police were able to tell the gunmen only that the British government was keeping the Iranian government informed, that deadline also passed, and negotiations continued.

Iranian Foreign Minister Sadegh Ghotbzadeh, who is visiting the United Arab Emirates, has refused to negotiate with the men who seized the embassy. "It is the duty of the British government to handle this drama," he said, "and the British will be responsible for everything that happens to our diplomats."

At a later press conference in Abu Dhabi, Ghotbzadeh blamed the embassy takeover here on the Iraqis, linking them with the Israelis and "American imperialism and Zionism." He said, "Iran will not give in to blackmail, either from the superpowers or a small number of terrorists."

Ghotbzadeh refused to see any comparison between the seizing of Iranian diplomats in London and the taking of American diplomats as hostages in Iran. "The occupation [of the American Embassy] in Tehran is a reaction to 25 years of suppression and killings in Iran," he said. "We condemn the occupation of our embassy [in London], which is totally in a foreign land of foreign people and has nothing to do with the issue."

Kensington road near the embassy today recalled televised scenes of crowds at the U.W. Embassy in Tehran. Hundreds of Iranian students from all over Britain marched and chanted their praise of Ayatollah Ruhollah Khomeini, Iran's leader, and hatred of President Carter.

They were heckled not only by anti-Khomeini Iranians but also by British college students singing patriotic songs and demanding that Iran "Free the Yanks."

In the evening, police decided to disperse the demonstrators and clashed violently with them in Hyde Park and Kensington.

Numerous arrests were made and a number of people were injured in the clashes, police said, but an exact breakdown was not immediately available. t

British Prime Minister Margaret Thatcher sent Iranian President Abol Hassan Bani-Sadr a message today informing him of "my deep personal concern" and pointedly stating the embassy takover here "constitutes an act of terrorism and an infringement of the immunity of diplomatic staff which the British government finds totally repugnant, and is acting firmly to counter." Bani-Sadr said in Tehran that he would rather let the hostages die as martyrs than give in to their captors' demands.

Scotland Yard Police Commissioner Sir David McNee told a press conference tonight that "the gunmen inside the Iranian Embassy must know that it is now within our power to meet all of their demands whatever our views on the rights and wrongs of their cause may be."

The police are trying to narrow the negotiations down to demands over which Britain has control: the future of the gunman themselves if the siege is ended and the already extensive publicity being given to their campaign for autonomy for fellow Iranians of Arab origin in Khuzestan. The province, on the Iranian-Iraqi border and referred to by the gunmen as "Arabistan," is the source of most of Iran's oil and the scene of periodic uprisings and sagotage of oil production facilities by Arab militants backed by Iraq.

"I appeal to them to remain calm," McNee said of the gunmen, who are believed to be monitoring media reports on radio or television inside the embassy. "Hasty action may cause even more suffering to their own people in Iran.'"

Overnight, in a telephone call to the gunmen from Abu Dhabi, Ghotbzadeh warned that for every Iranian Embassy hostage harmed, one of the imprisoned Arab rebels in Khuzestan would be tried and executed.

Durng the later telephone conversation with a BBC radio reporter here, the unidentified leader of the gunmen said Ghotbzadeh's message was "that he would not yield to our demands. I think he will regret that statement. After the deadline, we will kill everybody, all the hostages."

But in a statement read to the BBC by an unidentified hostage during the same telephone conversation, the gunmen declared. "We would like to assure the British public that the British hostages here will not be harmed."

Still later, shortly before the noon deadline, one of the British hostages, BBC news producer Chris Cramer, was released by the gunmen because he was suffering from serious cramps. He was hospitalized with a stomach disorder and ordered by police not to talk to newsmen. Cramer may have been the unidentified "badly injured" person who was feared yesterday to have been shot when the gunmen invaded the embassy.

There are at least five non-Iranians believed to be among the hostages. BBC news technician Sim Harris was with Cramer applying for a visa to Iran when the embassy was seized. Pakistani born British journalist Hashire Faruqi and Lebanses journalist Mustafa Karkouti were believed to have been trapped there while interviewing an Iranian diplomat.

London police Private Trevor Lock of the Diplomatic Protection Group was guarding the embassy when the invaders pushed him inside at gunpoint, and a British clerk or caretaker at the embassy also is believed to be one of the hostages.

The rest are believed to be Iranian diplomats and embassy staff headed by Charge d'Affaires G. Afronz, a stocky young man who recently told an American reporter that he was himself ready to be expelled from Britain or taken hostage in the cause of the Islamic revolution if it ever came to that.