Several hundred thousand Christians are expected to stream into Washington today for nonstop sessions of prayers, singing and sermons calling for spiritual rebirth in the country.

From sunrise to sundown, a "Washington for Jesus Rally" will gather on the Mall. Police say they expect traffic congestion in the vicinity to be oppressive, particularly during morning and evening rush hours and at noon when rally participants plan a procession along Constitution Avenue.

Metro spokesmen said delays on many bus and subway routes also are likely.

The purpose of the rally, which is drawing theologically conservative Christians from all over the nation, is to combat "the nation's drift from God," according to a rally organizer.

"The United States has gone off course spiritually and is consequently facing serious crises," said the Rev. John Gimenez of Virginia Beach, who conceived and helped plan the rally.

In a rally planned for youths and those arriving early for today's events, about 25,000 persons gathered at RFK Stadium last night to listen to music, hear preachers, and pray. The rally was held without incident despite frequent rainshowers that caused problems with sound amplification equipment.

Several hundred persons had to be turned away from an overcrowded women's meeting at Constitution Hall held yesterday afternoon in connection with the rally. But nobody seemed to mind being shut out. While the crowd waited hopefully on the steps, impromptu preachers among them grasped the opportunity for quick sermons.

"We must repent," shouted Richard Swanson of Pasco, Washington, standing under a red and blue banner that proclaimed: "Jesus first and Always"

The crowd murmured its approval: "Praise God" "Praise the Lord!" "Thank you Jesus!"

For today, transportation officials urged motorists and Metro bus and subway riders to avoid the Mall area.

A procession of about 100,000 rally participants along Constitution Avenue between Seventh and 14th streets NW is set to begin at noon and will continue for about 3 1/2 hours.

Eastbound lanes of Constitution Avenue will be closed during the march, and police say they may have to close the westbound lanes as well. Also, 14th Street between Constitution and Independence avenues will be closed to all traffic except metrobuses during the march, and all streets crossing the Mall between Seventh and 14th will be closed all day.

Between 3 a.m. and 5 a.m., Metro will operate a special shuttle train between the RFK Stadium-Armory station and the Smithsonian station to transport riders to the 6 a.m. service on the Mall. No fares will be charged because rally organizers have already paid $20,000 for the service.

Also, charter buses will be unloading rally participants at the Mall and transportation officials said they expect traffic tie-ups in the area.

Metro officials said riders are encouraged to avoid if possible the use of the Metro Center station and others, such as Smithsonian, Federal Triangle, L'Enfant and Union Station.

Passengers are encouraged to make their entire trips on the Red or Blue Lines, and walk from stations to their downtown destinations rather than transfer today, officials said.

Metrobuses normally serving the Mall area will be rerouted beginning at 11:30 a.m., about a half hour before the march. Bus routes affected include Nos 32, 34, 36, 52, 60, 70 and the M8.

Many bus routes between Washington and Alexandria will originate or terminate near the Mall will also be affected, as will the inbound No. 16 bus. The outbound No. 16 will use its regular route.

The procession is expected to end about 3:30 p.m. Meanwhile, speeches and musical events will continue on the Mall until about 6 p.m.

However, officials said they expect the crowd to begin dispersing during the evening rush hour which may create major traffic tie-ups in and around the Mall area.

Gimenez and other sponsors of the nondenominational rally have denied charges of more liberal church leaders that the Washington for Jesus events have political overtones.

A position paper, issued earlier this year denounced homosexuality, abortion, and excessive government spending, and complained that "our poor have become the perpetual wards of the state" and that "our government has aided our enemies and destroyed our friends."

The statement also asserted that the nation's elected officials "are first servants of God, then servants of the people."

The Rev. Bill Bright, director of Campus Crusade and one of the leaders of the rally, has said that this statement, which he signed in January, has since been withdrawn.

Critics, however, point out that a little-publicized aspect of the rally included visits by participants yesterday to members of Congress.