Led by a blue pickup truck from which a scratchy sound system blared hymns, about 200 persons climbed the hill to the grassy slope opposite the north porch of the Washington Cathedral late yesterday afternoon.

On the grass, two priests stepped forward to lead the group in a brief evening prayer service, using the 1928 version of the Book Of Common Prayer.

The alfresco service by the traditionalist Prayer Book Society was the latest skirmish in the battle over the prayer book, a conflict that the Episcopal Church thought it had settled five years ago.

The society had written Bishop John T. Walker last spring for permission to hold an evening prayer service at the cathedral during the society's annual meeting here this weekend. The society said it would use the old prayer book, which no longer is sanctioned by the church.

Walker replied that the group would have to use the prayer book adopted at the church's General Convention in 1979. The new book retains, for optional use, many prayers found in the former one, including the service of evening prayer.

But this compromise was unacceptable to the Prayer Book Society, which wants nothing to do with the new book. Instead, the Rev. Jerome F. Politzer, who heads the society, told his followers they had been banned from the cathedral.

Yesterday's outdoor service was the society's protest against Walker's decision, although there was no mention of the incident during the service. At the appointed place in the liturgy, Politzer offered the usual prayers for "our bishop, John."

Updating rites of worship to reflect changes in theology and the contemporary situation and language of the worshiper has been difficult for every church that has undertaken it.

Leaders of the Prayer Book Society, formerly known as the Society for the Preservation of the Book of Common Prayer, claim "hundreds of thousands" of followers, although fewer than 200 turned out for the annual meeting.

Walker said that the clergy of the diocese, as well as the national church's House of Bishops, have backed him in dealings with the Prayer Book Society.