U.S. Capitol police yesterday arrested 242 singing, flower-waving Christians as they knelt in the Capitol Rotunda to pray as part of a two-day protest against the Reagan administration's nuclear arms policy in general and the proposed MX missile development in particular.

The protesters, many of them clergy, were arrested for "unlawful conduct"--District statutes forbid demonstrating in the Rotunda--and taken out of the Capitol in handcuffs, to the applause and cheers of supporters.

Unlawful conduct is a misdemeanor, punishable by a fine of $500 or 6 months in jail or both.

Danny Collum, a spokesman for the protest organizers, said that "the majority intend to plead guilty."

Those arrested were taken for processing to a special area set up in D.C. Superior Court. Most expected to spend the night in jail, Collum said.

The protest was organized by a tiny evangelical Protestant organization called Sojourners, but the nonviolent demonstration included a wide spectrum of church people, with Roman Catholics apparently in the majority.

Shortly before noon, more than 1,000 demonstrators of all ages and representing a broad spectrum of Christian churches gathered on the west steps of the Capitol to listen to songs and speeches and wave peace banners.

Their applause and cheers repeatedly interrupted Del. Walter E. Fauntroy (D-D.C.), who thanked the demonstrators for raising "a moral argument" over development of the MX. The path to peace, Fauntroy said, "is disarmament, not nuclear weapons escalation."

"There is a conversion happening in the churches," said Jim Wallis, evangelical theologian who founded and remains the spiritual leader of Sojourners. "It is a conversion happening for the sake of peace."

Earlier this month the nation's Roman Catholic bishops adopted a widely debated pastoral letter that stopped short of branding any use of nuclear weapons morally unacceptable. Later this week in California, a disparate assembly of Christians--Roman Catholics and Protestants from the evangelical right to main-line and liberal left--will gather for an unprecedented conference on peace.