Nester Pereira, a foreing exchange student from Nicaragua, was taking off his shoes outside the Islamic Center on Massachusetts Avenue NW when he glanced to his left and saw a black man holding a rifle and shouting "Come here!" at an unidentified "fat man."

"I thought he was going to kill the fat man because he kept shouting at him," said Pereira, who froze as we watched the scene early yesterday afternoon when terrorists, who later identified themselves in a phone conversation as part of the Hanafi Muslim community, took over the center's mosque and as many as 15 hostages.

But the rifleman didn't shoot, Pereira and other foreign exchange students who witnessed the scene while on a tour said.

Instead, he forced the man to go inside the center's office along with another student and the student tour group leader, then slammed the door.

"We have 15 hostages, that's all the you need to know," someone who answered the phone at the center shortly thereafter told a Washington Post reporter. Then he hung up.

In conversations with a Washington Post reporter, one of the gunmen at the mosque sounded remarkably calm, in marked contrast to reports from the B'nai B'rith headquaters.

He spoke and referred to the caller as "sir." He said it was "futile" for the reporter to keep asking questions about his intentions. "You'll know soon enough," he said coolly.

In later phone conversations the terrorists identified themselves as being "American Muslims" with the Hanafi Muslim community. A newspaper reporter could hear muffled sounds - apparently the hostages talking in the background.

"Everything's fine," said the man who answered the phone about 6:30 p.m. "We're all having coffee and tea and a nice chat, but heads will roll and people will die unless we get our demands."

He said the operation at the mosque was "very closely co-ordinated" with other terrorist actions in the city and that "we're very prepared" wiht armaments should the police attack.

The man on the telephone said an American Muslim named "Hamaas" was the leader of the coordinated terrorist activity throughtout the city. He gave no further identification.

The scene at the Islamic Center, where events began to unfold about noon, according to police, was part of an extraordinary drama that engulfed several sections of the city throughout the afternoon and into the night.

Assistant D. C. Police Chief Burtell Jefferson, who was in command at the center, said the armed men there apparently were taking their orders from the gunmen at B'nai B'rith where another group of armed men also seized hostages, some of whom were injured.

As of 7:30 last night, no injuries were reported at the Islamic Center. Jefferson said he talked with the gunmen inside three times and that they were making no demands there by that hour.

The Islamic Center , located at 2551 Massachusetts Ave. NW, has been a center for Muslim religious and cultural activity for the last 20 years.

Its director, Dr. Muhammad Abdul Rauf, who was believed to be one of the hostages there, along with his wife, is a noted Muslim religious leader and scholar from Egypt. His wife and two of his four children - both believed to be in school at the time of the raid - live in the center.

According to Miraj Siddiqi, a leader in the local Muslim community, the Islamic Center has a staff of about six who run the office and oversee prayer services, which are held five times a day.

It is believed that some participants from the 12:30 prayer session may have been taken hostage along with the leader and members of the student tour.

The tour leader who was being held hostage was identified by the student s as the Rev. Robert Teasdell, 58, of New York City. He was leading the group of college students from Boston and Temple universities on a study trip arranged by his World Association for Travel, a New York company.

The student hostage was said to be Jose Luis Mora, 18, of Barranquilla, Colombia. The students said Mora's father runs a plastics factory there. Mora had been studying English, like the 40 others in the tour group, and was slated to return to South America in April, his friends said.

It could not be learned last night exactly who else was being held hostage in the Islamic Center. Edmond Howar, treasurer of the center, said that eight to 10 staff members usually were present during a normal working day.

At nightfall no injuries had been reported and the situation seemed basically stable.

About 10 of the students were inside the mosque when the takeover took place in early afternoon, but they left immediately and got away safely, some said during interviews in their downtown hotel afterward.

Doron Shwaic, a student from Israel, said he was inside when the tour group bus driver "came in excited and said, "Someone with a gun catch our leader (Teasdell), we're all in danger, they (the terrorists) are in the office and the police say we must leave."

All the students then left the mosque in safety , Shwaic said, but he said he did not know whether the half dozen or so persons praying in the mosque at the tme were able to leave.

Shaic said Teasdell had been inside with him but had gone out for some reason and then was captured.

Another student, Miriam Possenti of Venezula, said she was taking photos outside the center and remembered seeing two black men enter the office shortly before the take-over occurred.

When it did occur, she was inside in a small library room and heard a loud noise like a door slamming, she said. Then she went outside to safety with the others.

Pereira, the student who was taking off his shoes when he saw the terrorist with the rifle, said that the "fat man" who was captured outside had just left the office door, walking, and was himself shouing.

Police at the scene were tight-lipped about the information they had on the hostages and their actions. One police commander reportedly went into the building and tried to talk to the gunmen through the office door during the early afternoon.

By 3 p.m. a squad of rifle-carrying flak-jacketed special police had been posted around the building. later Metrobuses were lined up in front of the center to screeen traffic exiting from nearby Rock Creek Parkway in case of shooting.

By early evening as dusk fell, police had wheeled a large white Bethesda-Chevy Chase rescue truck with floodlights to the scene. There was no movement around the building.

The Islamic Center was completed in 1955 after a 10-year campaign started by a group of Muslim diplomats from several different countries.

Besides the first-floor offices where the hostages are believed to be held, the center included Rauf's family apartment , acombination library-museum. In a separate building, behind an ornate courtyard, is the mosque itself, which is set at an angle to the other buildings so that it faces east, toward the Muslims' holy city of Mecca.

Art works from several different Moslem countries fill the center and members of the Washington area's estimated 30,000 Muslims often attend services at the center during holy festivals.

In a telephone interview from New York yesterday, Teasdell's wife said her husband has run the non-profit travel organization for the last 25 years with the aim of providing low-cost tours of the U. S. for foreign students.

She said she had not been contacted by authorities but had learned of her husband's capture by a member of the tour group. "I suppose I should be more frightened than I am," she said. "But the reports said there hasn't been as much violence in the mosque. I'd hope there wouldn't be violence in a place of worship.

Mrs. Teasdell said her husband was an ordained Displines of Christ minister whose career has been spent dealing with young people.He once served as youth director for the National Council of Churches, she said.