While American hostages endured their 94th day of captivity in the U.S. Embassy in Tehran, more than 100 of their relatives gathered yesterday at Washington Cathedral to pray for their safe release.

In a solemn, 40-minute service, Episcopal Bishop John T. Walker seemed to voice the sentiments of many of the relatives, saying, "We are frustrated because we cannot move rapidly against those who hold our loved ones as hostages."

Several of the relatives, some of them wiping away tears, said after the service that they were resigned to the fact that the hostages would not be released soon. But many of them also expressed the belief that negotiations and other events in Iran will lead to eventual freedom for the hostages.

Louisa Kennedy, wife of Moorhead C. Kennedy, an economic officer held captive in the embassy, led a litany during the service and later said she views the hostage situation "with ever increasing hope.

"It's just a matter of time" until the hostages are released, said Kennedy, who lives in Washington. "There is a lessening of tension, a more constructive atmosphere. But don't ask me to be more specific."

Virgil Sickmann of Krakow, Mo., the father of Marine Sgt. Rodney V. (Rocky) Sickmann, an embassy guard taken hostage, said he hopes that as Iran's new president, Abol Hassan Bani-Sadr, assumes power, "maybe we can get our boys out.

"We just have to go back home, do our little thing, stay united and pray some more," Sickmann said. "President Carter is doing a good thing, as much as can be done."

Several of the relatives said the service helped boost their morale, but noted that State Department officials has not told them anything specific about a possible end to the hostage crisis during a day-and-a-half of briefings.

"They cannot tell us anything," said Janice Hassely, grandmother of Marine Sgt. Kevin J. Hermening, a 20-year-old embassy guard who is among the hostages. "They said that they now have more dependable people to talk to over there.

"I haven't heard anything from Kevin since I saw his picture on the television Christmas morning," said Hassely, who wore a white knitted armband with an American flag patch sewn to it. "He had a Coke bottle in one hand and a glass in the other. It was a beautiful sight. We did not expect that."

Yesterday's service was arranged by the State Department as one more effort to keep the plight of the hostages at the forefront of any consideration of the Iranian crisis.

Secretary of State Cyrus R. Vance and his wife Grace sat in the front row of chairs at the service, held in the great choir section of the cavernous cathedral.

Top officials from the Marine Corps and the Canadian and British embassies, a contingent of prospective Marine embassy guards from the Quantico Marine Base and some of the six American diplomats smuggled out of Iran late last month by the Canadian government were among the 400 who attended the service.

In a brief address, Bishop Walker read a portion of a letter that L. Bruce Laingen, the charge d'affaires in the American Embassy in Tehran wrote his teen-age son William shortly before Christmas. Laingen is not one of the hostages, but is being held with two others at the Iranian Foreign Ministry in Tehran.

Laingen said he was confident that eventually his captivity would end. "I remain convinced that there is decency in every human being," Laingen said, "and that it will yet prevail here."

Still, Laingen wondered in the letter, "Where is tolerance and compassion? Where is a recognition that perhaps the ultimate truth is not known here? Where is love?

"And I guess that is what has impressed me most in these five silent weeks . . . realizing what danger there is in hate, especially when it is fanned and encouraged as a virtual national tonic or dogma," Laingen wrote.

Laingen also praised the American resoluteness during the crisis, saying he believes "there is a firmness resting on a sense of restraint, restraint worthy of a country with power that should only be used as a last resort in a world that so desperately needs peace."

And then Louisa Kennedy led the congregation in the responsive litany: "We pray for those we dearly love who continue to be held hostage day and night."

"Stand by them by day and by night," the worshipers responded.