Demonstrations by Arabs and sympathizers, extra police guarding strategic points, and traffic problems around the White House will mark the scene of the historic signing of a peace treaty between Egypt and Israel at 2 p.m. today.

The demonstrations, to which Washingtonians have become accustomed, will disrupt traffic patterns around the White House.

Government officials expect large crowds of pedestrians from nearby office buildings and passing motorists hoping to catch a glimpse of history to contribute to the downtown congestion and possible confusion today.

Because of the ceremony, Pennsylvania Avenue will be closed off between 15th and 17th streets NW and Jackson and Madison places will be closed also from noon until 4 p.m. Buses that normally use Pennsylvania Avenue in front of the White House will be rerouted along I Street and Constitution Avenue during the fourhour period.

Police will close a portion of Pennsylvania Avenue because the ceremony, attended by Israeli Prime Minister Menachem Begin, Egyptian President Anwar Sadat, President Carter and 1,600 invited guests, will be held on the north lawn of the White House.

Arab demonstrators have scheduled protest demonstrations in front of the White House at Lafayette Square and the Ellipse, in back of the White House.

Deputy D.C. Police Chief Robert Klotz, commander of the special operations division, said the demonstrators will begin their march at Dupont Circle, and will proceed down Connecticut Avenue to K Street NW, where the group will split, some going to Lafayette Square and the others to the Ellipse, south of the White House.

For those who do not want to brave the expected crowds and the chilly 40-degree temperature, the ceremonies will be televised live by all three major networks.

A public ecumenical prayer service will be held at the Lincoln Memorial from 7:30 to 8:30 p.m. Speakers include Rabbi Stanley Rabinowitz, of Washington's Adas Israel Congregation, Dr. Muhammad Abdul-Rauf director of the Islamic Center here, the Rev. Theodore M. Hesburgh, president of the University of Notre Dame, and Ruth Carter Stapleton, the president's sister.

Begin, Sadat, and Carter will not attend the service, the White House said, although representatives of all three countries will be there.

At a state dinner later in the evening on the south lawn of the White House, about 1,300 guests will be serenaded by strolling violinists as the guests dine under a large orange and yellow tent.

Seated at round tables decorated with tablecloths of yellow and green branches on a white background, and centerpieces of forsythia branches and hurricane lamps, the guests will dine on beef sirloin, Columbia River salmon in aspic, spring vegetables and hazelnut giandja mousse for dessert.