Thousands of sightseers, workday Washingtonians and about 2,000 angry, shouting Arab protesters crowded into Lafayatte Park yesterday, all to witness -- for sharply contrasting reasons -- the historic Egyptian-Israeli peace treaty signing ceremony across Pennsylvania Avenue on the White House lawn.

Under unusually tight police security, sightseers with cameras and lunch-break government workers applauded politely. Behind them and surrounded by hundreds of riot-equipped police officers, Palestinian Arabs and their supporters shouted "Down with the treaty," "Sadat is a traitor," and "Long live the PLO."

Bright green-white-black-and-red flags of the Palestine Liberation Organization snapped in the breeze as demonstrators and sightseers alike bundled against the 45-degree air.

Shouts of the protesters carried across the park and Pennsylvania Avenue and could be heard on the north lawn of the White House. There, 1,600 dignitaries gathered with President Carter to watch the treaty signing by Israeli Prime Minister Menachem Begin and Egyptian President Anwar Sadat.

At one point, the protesters surged against a snow fence erected to contain them and knocked it down, but were quickly pressed back by mounted U.S. Park Police officers and the demonstrators' own marshals.

A few demonstrators wearing the red-and-white checked kaffiyeh head dress worn by PLO leader Yassir Arafat, filtered into the crowd of sightseers near Pennsyslvania Avenue but quickly were pulled out by Park Police.

In the minutes following the 2 p.m. ceremonial signing, the shouting and applause built to a crescendo. The amplified speeches of Carter, Begin and Sadat combined with the Arab and English chanting of the protesters, the bells of nearby St. John's Church pealing in celebration of the treaty and the periodic thunder of airplanes roaring toward National Airport.

Security was tight throughout the White House area. Pennsylvania Avenue and adjacent streets were closed to traffic.

Hundreds of Park Police, D.C. Police and uniformed Secret Service officers with helmets, riot sticks and gas masks formed protective lines across Lafayette Park, Pennsylvania Avenue and even the lawn inside the ornamental iron fence at the White House.

Secret Service agents stood on the White House roof, scanning the crowd with binoculars. Other agents were scattered through the crowd in the park.

Police officials, concerned that disruptions might be triggered in the highly emotional atmosphere surrounding the treaty signing, were fully braced for trouble. It never came.

A radio reporter was struck in the mouth by a demonstrator, and one protester said he was struck by a by-stander. Police reported no other injuries.

The demonstration, sponsored by the Organization of Arab Students, began at midday at Dupont Circle. Protesters marched down Connecticut Avenue and would through downtown before arriving at Lafayette Park.

Habib Fakhouri, 40, called the Egyptian-Israeli treaty a "mockery" that ignores the rights of Palestinians to regain control of the Israeli-occupied West Bank, Gaza and East Jerusalem.

Other protesters called for armed victory by the PLO over Israel so that the several million Palestinian Arabs who left that country when Israel was formed in 1948 may return.

"I want to return to my home," said one protester, who refused to give his name. "... I want to go back and smell the oranges."

James Lockett and his wife, Reita, both of McMinville, Ore., were among the sightseers in the park. "We think it's great, it's really exciting," he said. "Now we will have something to take back to our children."

Also contributing to this story were Washington Post Staff writers Alfred E. Lewis and Stephen J. Lynton .