It had, until recently, been one of those quietly classy streets lined with town houses -- currently valued upwards of three quarters of a million dollars -- where contentment fairly glows from the polished brass door knockers.

On East 65th Street in Manhattan, money does not shout; it purrs like a sleek and satisfied cat. Elegance, tranquility and respectability are the bywords.

Or were, until the new neighbors arrived.

"At one time we were known as the center of New York's Gold Coast. We were being considered for designation of a city landmark," said Irwin Glass of 114 E. 65th St.

He spoke with the wistful air of one who senses that an era may have passed. He is probably right.

Things simply have not been the same on East 65th Street since the Palestine Liberation Organization bought number 115 in January and former President Richard Milhous Nixon moved into number 142 in February.

Now there are sightseers traipsing up and down the two-block stretch between Third and Park Avenues, between the PLO headquarters and the Nixon abode.

On the sidewalk in front of the PLO's soon-to-be-occupied house, a wooden booth, painted blue, had been erected by the New York City Police Department. It is there so that the officers assigned to guard the premises around the clock on two-man patrols can take turns keeping warm.

As for the house itself, the red brick Georgian structure has taken on a forbidding air. The front door has been replaced with an electronically operated slab of steel. It is flanked by two cameras that jut out over the sidewalk.

Yards of aluminum mesh, like shiny chicken wire, cover the side garden, the building's rear and all the gracefully arched windows.

From across the street the place looks not unlike a very large, well-fortified chicken coop with a toll booth in front of it.

A lot of people mistake it for the Nixon residence a block and a half away.

"Passersby often stop and ask me, 'How is Mr. Nixon doing?'" said an officer named Michaelessi with a wide grin, as he stood outside the house the PLO purchased early this year for close to a million dollars in cash.

On a recent evening, an East 65th Street resident who was walking her pedigreed dog observed two young men gazing in awe at the house from across the wooden police barricade that permanently guards its curb. "Is Nixon that frightened?" one of the men was heard to wonder aloud.

Not everyone is confused about where Nixon lives, however. To the dismay of the neighbors, two dozen hairy, unkempt, cheerful Yippies, calling themselves The Committee to Exchange Milhous for the Hostages, turned up outside Nixon's place in late February.

They waved placards -- one said "Draft Tricia" -- and chanted, "Send Nixon to Iran." The demonstration was organized by famed pie-thrower Aaron Kay, who has successfully pelted such public figures as California Gov. Jerry Brown and Sen. Daniel Moynihan (D-N.Y.) in the past and is known to be stalking Nixon.

While the Yippies whooped it up on one end of the block, having been confined there by police, about a dozen well-dressed, middle-aged counterdemonstrators marched decorously in a circle on the other end. One of the loyalists carried a banner that said, "Welcome to Our New Neighbors, President and Mrs. Nixon."

All of the hoopla went unnoticed by the Nixons, who were not home at the time. But then, the PLO house has not even been occupied yet and it has already been picketed twice -- by the Jewish Defense League and by a group protesting the persecution of Soviet Jews.

In general, the Nixon's arrival has been treated by the denizens of East 65th Street with good-humored resignation. As novelist Nancy Winters observed in a recent essay, "It's the most excitement we've had round here since manhole covers blew outside the Sign of the Dove," an elegant restaurant on the corner of East 65th and Third Avenue.

Neighbors have spotted Nixon having his hair cut at the Regency barber shop and shopping for baby clothes with his daughter on Madison Avenue.

Last month, when the Nixons first visited their 12-room, $750,000 town house, located next door to banker David Rockefeller's home, a small group of neighbors turned out to say hell and cluck sympathetically as the new arrivals sidestepped several mounds of uncollected garbage lining the curb -- a common New York hazard, even on the city's most elegant blocks. t

Reaction to the PLO, however, has been anything but sympathetic. The community has been in a dither since mid-January, when it became known that Zehdi Labib Terzi, the PLO's permanent representative to the United Nations, had paid $950,000 cash for the residence, which Terzi plans to call "Palestine House."

Freely admitting that the money had been supplied by sympathetic supporters of the Palestine cause, including Saudi Arabia, Kuwait and the United Arab Emirates, Terzi announced that he, his wife, Widad, and their two teen-age children would be moving in as soon as the place was refurbished.

Part of the house, Terzi indicated, will be used as office space by the PLO, which is losing its lease on Park Avenue.

"We had heard rumors for some time that the house was being sold," said on longtime resident. "First someone said it had been purchased by a comedian. Then the story circulated that it was a black comedian. But we never expected this." (It was reported that the house had been shown to comedian Bill Cosby.)

Christine Schubert of 118 East 65th St. and her husband, Bernard, an investor, are spearheading a campaign to oust the PLO. They have organized meetings, circulated petitions, hired legal counsel and a public relations consultant, collected funds and demanded a hearing before the community planning board, charging that use of the building by the PLO is a zoning violation.

"We are not anti-PLO," Schubert insists. "This is purely a security issue. It's not that we think that they are going to drop a bomb on their own house. It's just that we're afraid someone else is."

Schubert said she understands that the PLO has been trying to improve its image lately, and would naturally be attracted to a neighborhood like East 65th Street. But, she added, "You have to earn respectability. You don't get it by buying a building on a so-called respectable block. That is only money," she said with a sniff, "and everybody knows they have plenty of money."