Officials of Brigham Young University said today that they will continue, and may even speed up, construction of a $15 million Mormon study center here in the face of increasingly bitter opposition by a coalition of orthodox and secular Jews that has warned of growing conflict over alleged Mormon efforts to convert Jews.

Despite the appointment last night of a Cabinet committee to study the planned Mormon center and a decision today by the orthodox Agudat Yisrael Party to submit a parliamentary motion of no confidence over the issue, university spokesmen said no responsible government official had even suggested to them that construction of the seven-story structure on the slopes of Mount Scopus overlooking Jerusalem should be halted.

David Galbraith, director of the Mormon study center, said that the church conducted no proselytizing or missionary activities in Israel.

Attorney General Yitzhak Zamir said he doubted that the federal government could intervene in the matter to overrule the municipal government of Jerusalem Mayor Teddy Kollek. Kollek has said he has no intention of stopping the construction, and warned that the controversy could harm U.S.-Israeli relations.

However, the dispute intensified today, with Mormons reporting to have received telephoned threats against their lives and secular Jews joining an anti-Mormon campaign that until now had been confined largely to the orthodox Jewish community.

Leaders of the newly formed orthodox-secular coalition, the Public Committee to Stop the Mormon Missionary Project, said at a press conference today that they have "substantial evidence" suggesting that the Mormons do not intend to abide by a written pledge to the Israeli government not to use the study center for missionary activities.

Calling the new center an "outrage, disgrace and shame to the people of Israel," Rabbi Solomon Sharfman, chairman of the Citizens' Committee for Jerusalem, said, "We don't want others to belittle our faith by attempting to win Jewish souls to another faith."

Rabbi Simha Kook, chairman of the Chief Rabbinical Council, said that the Mormon faith, known formally as the Church of Jesus Christ of Latter-day Saints, is built on missionary activity, and that "Mormons hold Jews responsible for killing Jesus."

Kook added, "They are interested in converting Catholics, Protestants and Moslems, too, but particularly Jews. . . . If we were to build something like this in Salt Lake City, believe me, there would be more opposition than this." Salt Lake City is the seat of the Mormon Church and Brigham Young is in Provo, Utah.

Representing the secular wing of the protest movement, Eli Ben-Gal, a historian at the Museum of the Diaspora and a representative of the kibbutz movement of the Socialist Mapam Party, said that the Mormon center should be of particular concern to secular Jews.

"If the Mormons want to pray, they have plenty of churches in Jerusalem. But we cannot accept the basis of Mormon fundamentalism -- to come back to Christ," Ben-Gal said. Among Jews, the nonreligious are the chief targets of Mormon missionaries, he said.

Mordecai Rotenburg, a Hebrew University sociologist, said he had studied proselytizing religious sects in the United States, including Jews for Jesus, the Unification Church of the Rev. Sun Myung Moon and Hare Krishna, and was equally worried about the Mormons.

"I'm tired of losing people," Rotenburg said. "This is an extensive fear. Thousands of Jewish kids comprise the Jews for Jesus. That's enough for me."

Leaders of the protest groups were vague about how successful they thought the Mormons' missionary activities had been in Israel.

Moshe Dann, spokesman for the Yad Lachim Jewish Orthodox lobby, said in an interview that he knew of no more than 10 Jewish converts to the Mormon faith in the last 15 years, although he stressed that there could have been more.

At today's press conference, Dann introduced a youth who identified himself as Shalom Maor and said he had worked for 1 1/2 years at a Mormon-owned ranch in Florida. There, he said, he learned that the Mormons' "only purpose is missionary."

Questioned closely by reporters, the youth said he had been raised as a Christian in Texas and had recently immigrated to Israel, where he is in the process of converting to Judaism.

Another speaker, Rivka Sertvenik, said she was born to a Jewish family in Tucson, Ariz., but was converted to the Mormon faith while attending the University of Texas and remained in it until 1972, when she immigrated to Israel and renewed her Jewish faith.

"We were told every Mormon is a missionary and were taught to be a good neighbor because by being a good neighbor you attract people to the faith," Sertvenik said.

Galbraith, the Mormon study center's director, said tonight that during the 17 years Mormons have operated a Near East center here, he is aware of only two Jews who have been converted to the Mormon faith, and both of them had come from abroad to be baptized in the Jordan River.

"We have not had a proselytizing program here. We make a sharp distinction between our church activities and our university activities, and we do not engage in missionary activities in any country where we do not have permission to do so," Galbraith said by telephone.

Dann, however, claimed to have "substantial evidence that the Mormons were in violation of previous commitments they gave to officials here not to proselytize. So any commitments given now are subject to question."

He cited published letters written in 1977 by Galbraith to Brigham Young University officials in Utah outlining Mormon missionary plans here. Galbraith acknowledged that missionary activity had once been planned for Israel, but he said it had been dropped before construction of the new study center began.

Calling 1977 an "exploratory period" for Mormon educators here, Galbraith said, "We make no excuses about our missionary activity worldwide, and in that stage we were exploring various possibilities. But we changed our plans when even those activities would not be acceptable here."

He noted that last August, Jeffrey Holland, the president of Brigham Young, gave a written pledge to the Israeli parliament's Interior Committee that Mormon students would not be permitted to proselytize here and that if they did they would be sent home.

Galbraith added, "We propose to continue on at the present rate of construction and even to speed up if we can. We welcome discussion while we continue construction, but we are not going to stop work for even a minute." He said the target date for completion of the 200-student center is mid-1987.

Galbraith said that he had received numerous threats in recent weeks, the most recent tonight when an anonymous caller told him that if he left his home he would be killed.