Religious dissent is widespread and growing in the Soviet Union, a British specialist in Soviet religion told the Helsinki Commission on Security and Cooperation in Europe here this week.

"Official restrictions and periodic persecution remain fairly constant, but seem to have somewhat less effect as the various dissent movements develop, diversify and receive increasing support from the West," Peter Reddaway, lecturer at the London School of Economics, said Wednesday.

Although soviet religious legislation violates the "spirit and letter" of the 1975 Helsinki accords, the document "has given new encouragement to Soviet believers," Reddaway testified.

"If Western governments press the believers' case persistently in the years ahead, there is reason to hope for a real, gradual and extremely reluctant retreat by the Soviet government," he said.

The commission, an independent agency of 12 members of Congress and three executive branch officials who evaluate and encourage compliance with the 1975 agreement signed by 35 nations, held hearings Wednesday and yesterday on the human rights guarantees of individual "freedom to practice and profess . . . religion or belief" and "of equality before the law" for minorities.

Rep. Dante B. Fascell (D-Fla.) is chairman of the commission.

"Perhaps it is possible to understand repression of political opponents, the dissidenst, but it is hard to understand the persecution of people who are not opposed and who even are defenseless," Yevgeni Bresenden, a Pentecostalist said.

Bresenden, a 36-year-old electrician, served three years in prison in the 1960s for his religious beliefs. He succeeded in September, 1975, in becoming the first Pentecostal applicant to emigrate from the Soviet Union on the grounds of religious intolerance.

In chronicling the tribulations of some of the 200,000 to 400,000 Pentecostalists. Bresenden contended that the Helsinki accords are violated by the government's "rules of registration" for religious societies.

The rules forbid children under 18 to visit prayer meetings, the convening of women's meetings or any special prayer, literary, musical or work groups or clubs, and the collection of money to aid the ill, old or imprisoned -- all against the believtrs' consciences.

In addition, he said, the government frequently does not, as the rules require, supply a house of worship free of charge.

"Believers do not have the right to choose their presbyter (minister), unless his candidacy is approved by an atheist -- the representative of religious affairs," Bresenden said. "This situation -- in which the selection of presbyters depends on the will of atheists -- is interference in the right of people to choose an organization for the satisfaction of their spiritual needs."

Sister Margaret Traxler, cochairperson of the National Interreligious Task Force on Soviet Jewry, urged that the Helsinki commission press for freedom of movement for believers and relaxation of restrictions on the registry of houses of worship and against teaching religious education to youths under 18.

"These three objectives are reasonable and moderate but they would be seen as tremendous blessings to believers in the U.S.R." she said. "If we succeed in this endeavor, it will truly be a 'victory of justice.'"