From outside, things looked as calm as usual yesterday at the Embassy of the Islamic Republic of Iran at 3005 Massachusetts Ave. NW.

But behind the immense dark wooden front doors, officials and aides bustled about, monitoring the tense situation in the Iranian capital of Tehran where Iranian students continue to occupy the American embassy and hold dozens of employes hostage to their demand for the extradition to Iran of ousted Shah Mohammad Reza Pahlavi.

Iranian officials also had beefed up security yesterday in and around their building here following several crank calls and three bomb threats in recent days, apparently backlash to events in Tehran.

Officials were in constant contact by telephone yesterday with both Tehran and the U.S. State Department in an effort to resolve the American embassy occupation peacefully, according to Mansour Farhang, cultural and scientific counselor at the Iranian embassy.

A small but steady trickle of visitors, most of them Iranian nationals, came to the embassy throughout the day, some asking about developments in Tehran. Visitors waited in unadorned, sparsely furnished rooms, rooms stripped of the sumptuous furnishings of the shah's regime by the Moslem followers of Ayatollah Ruhollah Khomeini.

While thousands of students demonsdtrated in Tehran's streets yesterday in support of the American embassy takeover, Iranian students here were quiet. The streets of Washington frequently rang with the revolutionary shouts of masked Moslem and Marxist Iranian students in the latter years of the shah's reign.

But yesterday, there were no such demonstrations. "We've been monitoring the situation, but so far there's been no activity," said Dick Bottorff, director of the mayor's command center.

The FBI and Secret Service confimed that the Iranian Embassy received three telephoned bomb threats in recent days. The latest occurred at 12:30 p.m. yesterday. An anonymous male caller said a bomb was timed to go off at 6 p.m. No bomb was found.

Members of the D.C. Police Department bomb squad and the uniform division of the Secret Service responded by searching the embassy building, but they found nothing.

Embassy Charge d'Affaires Ali Ashgar Agah said embassy employes had beefed up internal security as a result of the telephone calls. Secret Service officials would not say what, if any, additional protective measures they had taken, but termed security "adequate."

An FBI agent termed the phone threats as "general crank calls . . . It's a harassing thing. It seems to be a fad."