Unusually tight security was clamped on downtown Washington yesterday in the wake of the Soviet Aeroflot office bombing as federal and local police agencies also coped with a rash of demonstrations, bomb threats and the arrival of western hemisphere dignitaries for the ceremonial signing of the Panama Canal Treaty.

Police helicopters whirred overhead and Secret Service sharpshooters perched on rooftops. Demonstrators and uniformed police ebbed and flowed between the Capitol, the State Department and the White House. The demonstrators were variously protesting the canal treaties and the presence of many of the visiting Latin American heads of state whom they consider to be military dictators.

Several hundred Secret Service and State Department agents were deployed to guard the approximately 50 foreign presidents, prime ministers, diplomats and members of their families as they were whisked in limousines from place to place throughout the day.

"This is the largest security event of this sort (involving foreign dignitaries) since President Kennedy's funeral in 1963," said Leo Crampsey, chief of the foreign dignitaries division of the State Department office of security.

Police were summoned to investigate at least six bomb threats in downtown buildings including the Washington Monument, the Hyatt Regency Hotel and the Capitol Hill Quality Inn.

The monument was evacuated at 5 p.m. shortly after the Associated press received a telephone call warning that the monument was a bomb target. Park police then began a sweep of the structure but had found nothing by early evening.

Police and employees at the two hotels also conducted room-by-room searches, but discovered nothing.

One point of intensified security was the Pan-American Union building at 17th Street and Constitution Avenue NW, headquarters of the Organization of American States where the visiting dignitaries gathered in the late afternoon.

Up to 1,500 demonstrators, most of them protesting what they say is political oppression in many Latin-Amercian countries, also converged on the ornate building during the afternoon.

A cordon of about 100 park police officers backed by 50 D.C. police patrolmen 50 on scooters and about 40 Executive Protective Service officers blocked the protestors from the front of the building, causing many to go around to the rear along Virginia Avenue.

Seventeenth Street in front of the building was closed to traffic. About 25 Secret Service agents kept a wary watch on stragglers crossing the Ellipse near the building. Manholes on 17th Street had been sealed earlier in the day.

"This was a total security plan," said D.C. emergency preparedness office chief George Hodericks. "There was nothing routine about it."

All the various demonstrations throughout the day were peaceful, and no arrests reported.

The protesting groups ranged from the Emergency Coalition to Save the Panama Canal and fundamentalist radio preacher Carl McIntire's March for Victory, to groups of Dominican Republican Nationls, to an umbrella organization called the Coalition Against Recression in the Americas.

Various factions of the coalitions broke off from the main groups from time to time and marched back and forth between the Capitol and the White House-OAS area near the Ellipse.

About 150 protestors applauded Reps. Larry McDonald (D-Ga.) Robert K. Dornan (R-Calif.) and other conservatives at the Capitol as they called for repudiation of the canal treaties.

McDonald called the treaties "not a Democratic giveaway or a Republican giveaway - it's bipartisan treason."

Dornan, a former California television talk show host, yelled into the microphone on the east steps of the Capitol that Panamanian President Omar Torrijos is "skimming money from whorehouses," has "Swiss bank accounts" and is "unstable, immoral, venal, a leftist dictator of the highest order."

At the other end of the political gamut, demonstrators organized by the Coalition Against Repression in the Americas, gathered to condemn the presence here of what they say are Latin American military dictators primarilyof the right wing.

Speakers at a Lafayette Park rally told several hundred protestors that President Carter contradicted his highly touted human rights policy in inviting the Latin American leaders here for the treaty signing ceremonies.

"From now on, Jimmy Carter, you have given us the signal of what our attitude must be towards your administration," said Sue Borenstein of the Chile Solidarity Movement. "This administration is all lip, all words."

Present at the rally was Isabel Letelier, widow of former Chilean ambassador to the United States Orlando Letelier who was assassinated here last year. Letelier had served in the government of Marxist President Salvatore Allende before the present rightest regime of Augusto Pinochet. Most of the other demonstrators appeared to be Americans.

Other speakers denounced what they called the repression and torture practiced by many of the dictators in South America, especially in Chile, Uruguay, Paraguay and Bolivia.