UN birthday in Manhattan: as much a party as business. Protecting diplomats involves humor and patient police work

THE 40th anniversary of the United Nations is a grand party, according to some. It's strictly business, say others. But however you define it, the anniversary of the UN is a very visible, very busy affair in midtown Manhattan.

It means ubiquitous sirens and motorcades -- and roads closed to regular traffic.

It includes a bevy of receptions in the evening, with the usual political intrigue. No, Prime Minister Rajiv Gandhi of India will not be honored today at Gracie Mansion by Mayor Edward I. Koch, as had been scheduled earlier. Mr. Gandhi canceled his appearance after Mr. Koch canceled a trip to India in protest of India's urging that Yasser Arafat of the Palestine Liberation Organization (PLO) be invited to address the UN.

The anniversary also means that police officers are posted in droves near the UN and the Waldorf Astoria, where many foreign dignitaries are staying. There are marksmen with high-powered binoculars and firearms on the roofs of several building at the UN. And the New York City Police Department (NYPD) has had a temporary headquarters since September 17, at 45th Street and First Avenue.

Early Saturday morning, the New York City police officers manning the headquarters near the UN were startled when the ground under them began to shake and rumble.

``At first they thought it was the PLO, out for reprisal,'' says Assistant Police Chief Gerard J. Kerins, who is commanding the NYPD's portion of the massive security operation that will safeguard nearly 80 heads of state and 150 foreign ministers in town for the ceremonies this week.[ufbj,15p]UNUN But the rattling of Saturday morning was an earth tremor felt throughout the metropolitan area.

So far, things are running smoothly in New York City. Despite the motorcades, traffic has been relatively unsnarled, says Transportation Commissioner Anthony R. Ameruso. A bomb scare on Monday near a motorcade route was handled with dispatch.

In addition to the NYPD, the United Nations Security and Safety Service, the Secret Service, the Department of State, and the Federal Bureau of Investigation are involved in security during the UN meetings.

Duty at the UN can be boring, says Officer Mike Scanlon, but he enjoys his duty when it involves people. ``I saw Richard Nixon last night,'' he said. ``President Reagan will be there Wednesday.'' But he is clearly impressed by more than just the foreign dignitaries.

``I saw the guy who plays Renko on Hill St. Blues. And Bernadette Peters! There goes the president of Pakistan,'' he says, pointing to a motorcade as it heads up First Avenue.

At Ralph Bunche Park, Lt. Phil Franzese confirmed that there was a bomb threat below 42nd street in an area where a pro-Nicaragua demonstration had taken place earlier. There was a suitcase under a bench, said Lt. Franzese.

A member of the Yugoslavian delegation was trying to walk to the park when he encountered the police barricade.

``Can I go over there?'' he asked.

``No,'' replied Franzese, ``there is a bomb threat.''

The Yugloslavian man stood for a minute, and asked again. ``Can I go over there to take a picture?'' he asked, pointing across the street.

``No, you can't, this area is frozen because of the bomb threat.''

``Oh, there really is a bomb threat,'' the Yugoslavian said. ``I thought you were joking.''

``That's why we are moving back,'' said Franzese, who normally has a desk job. He pointed to the police department's robot that would approach the suspicious package. The robot eventually reached the suitcase picked it up. It contained old clothes.

Once on UN territory, which is not a part of the United States, UN security becomes responsible for the safety of guests. Federal and local law enforcement officials may enter, but they are guests and no longer working.

Secret Service agents are easily recognized by their conservative dress, the lapel buttons, and the headphone that looks like a hearing aid. They are often talking, seemingly to themselves.

High technology is also playing a part in the security. Police can do a five minute overview of the entire area around the UN and the Waldorf Astoria using handheld television cameras at strategic points. Digital computers help track motorcades. Cellular telephones are used for confidential, uninterrupted communications.

And a state of the art IBM computer in the makeshift headquarters puts together information on traffic, demonstrations, what dignitaries are currently in town, and some intelligence information.

Surprisingly, costs for the NYPD are considerably less than they were in 1960, when the United Nations last hosted a similar celebration. So far, the police have spent less than $380,000, says Kerins. In 1960, the total cost for 26 heads of states over a three-week period was $6 million.

``We can come in in the morning, and ask the computer what is going on the next few days,'' says Lt. Joe Congelosi. ``It helps us to make manpower decisions. It saves a lot of planning time, and a lot of wasted manpower.''

Bob Snow of the Secret Service can't talk about the number of agents assigned to New York City while the celebrations are ongoing. But he does say it is the largest operation to date, bigger than political conventions or the Olympics, for example. And it sometimes means unusual duties.

``We try to fit into the lifestyle of the person we are protecting,'' says Mr. Snow. That might mean running in Central Park, like agents did with the Nicagarguan President, or going horseback riding or boating.

Different heads of states have mixed both UN business and other chores. Austrian Chancellor Fred Sinowatz presented a three-hour video tape titled ``Opera for Africa'' on Monday. He hopes to sell television rights to the show in order to raise $1 million in famine relief and rehabilitation in Africa.

Chancellor Sinowatz has also gone to Mayor Koch's Sunday evening reception and visited Wall Street, where he talked with Henry Kaufman, the chief economist of Saloman Brothers.

Prime Minister Nakasone of Japan has visited Sagamore Hill, Teddy Roosevelt's home, and also visited a classroom at Bronxville Elementary School in Westchester County on Monday.

The President of Finland, Mauno Koivisto, has been busy with meetings, but he will find time to go to the Metropolitan Opera tonight.