Thousands of antiabortion activists are expected to rally at noon Tuesday on the Mall and then march up Constitution Avenue to meet with members of Congress.

They will lobby members of Congress for legislation that would override the Supreme Court decision, handed down Jan. 22, 1973, that legalized abortion.

Abortion rights activists also plan to observe the 18th anniversary of the Roe v. Wade ruling. The National Organization for Women will have a small group stand along the route of the antiabortion parade and hold up 25 large panels displaying the signatures of more than 25,000 Americans who support abortion rights.

Although the March for Life is an annual event, this year's demonstration comes at a time of rising security concerns because of the Persian Gulf War.

Additional security measures could slow demonstrators down when they begin to enter the Capitol Hill buildings where representatives and senators have their offices, said Capitol Police spokesman Dan Nichols.

"I am sure they understand we are taking additional security steps," Nichols said. He said police are hoping that demonstrators will be "patient and understanding" about procedures put into effect for visitors entering the buildings in recent days.

Nichols and other police officials said they expect up to 50,000 demonstrators, based on figures supplied by March for Life organizers. Last year, police estimated that 75,000 people took part in the demonstration; organizers said there were 200,000 participants.

March for Life President Nellie Gray said the actual turnout this year will depend on several factors, including the weather and the situation in the Persian Gulf.

Gray expressed concern that public preoccupation with the war could distract attention from the antiabortion demonstration.

"March for Life is a single-issue focus," she said. "The focus has to be the babies killed within the domestic borders of the U.S."

During the rally, the demonstrators will be addressed by about 10 members of Congress who are against abortion, Gray said. She said that President Bush has been asked to speak to the group by telephone, as he has done at previous antiabortion demonstrations.

"We don't know yet if he will; we never know until the day of the march," Gray said.

After the rally on the Mall at Seventh Street NW, demonstrators will march up Seventh Street, turn right onto Constitution Avenue, march past the Capitol to First Street, turn right on First Street and march to the Supreme Court. The group will disband at that point and walk to the congressional office buildings in the area to visit their representatives.

Most participants will come from the East Coast, Gray said. But five busloads of demonstrators from Kansas and 20 busloads from Missouri are also scheduled to march, she said.

The theme for this year's demonstration will be "no waffling on the life principles," Gray said. She said the antiabortion movement has done "everything it can to educate politicians" that abortion is wrong.

Now the movement wants politicians to know that they can't get away with "waffling" on the abortion issue by trying to agree with both sides, she said. She added that those who do will be defeated at the polls.

Antiabortion activists will be pushing for three legislative proposals when they meet with members of Congress. These include an amendment to overturn Roe v. Wade, and laws to cut off public funding for abortion and identify "pre-born children" as citizens.

Metrorail will provide additional service for the extra riders headed for the rally and march.