Off-season Europe beckons with fall and winter bargains for budget travelers, Christmas shoppers and lovers of the fine arts. At this time of year, of course, it's too nippy to linger at an outdoor cafe (one of the joys of a summer visit); on the other hand, you won't bump into the huge vacation crowds.

"The best time to visit London is in the fall and winter," says Bedford Pace, spokesman for the British Tourist Authority, who points out that this is when the city's renowned theater, opera, concert and ballet seasons are in full swing. The same is true in Paris, Rome, Vienna, Copenhagen and other European capitals.

London, a lively theater town, boasts more than 60 professional theaters. Theater lovers on a week's visit can indulge themselves in a splendid binge of matinee and evening performances at comparatively low prices.

The top ticket price for orchestra seats at a hit musical in London, such as "Cats" and "Starlight Express," is about $25, half what you would pay for the best seats in New York. Half-price tickets for many shows can be purchased on performance day at a booth in Leicester Square, though admittedly rarely for "Cats" or "Starlight Express."

Elsewhere in London, the Royal Academy is honoring its first president, Sir Joshua Reynolds, with a comprehensive show of his portraits and other paintings, many of them borrowed from England's stately homes. The exhibit runs from Jan. 10 to March 30.

In Paris, the much-anticipated Picasso Museum has just opened. Occupying a restored 17th-century mansion in the Marais district near the Louvre, it houses 3,000 works that the painter bequeathed to France. Among them are many that have been exhibited only rarely or not at all. The museum also contains Picasso's personal collection of primitive art and works of his favorite artists.

Also under way is the Paris Opera's fall season, with seats ranging from about $5 to $50. A new museum of fashion, Musee National des Arts de la Mode, appropriate for the world's fashion capital, opens in January in the Louvre. The first exhibit, "Moments de Modes," will cover three centuries of fashion, from the 18th to the 20th. The Muse'e des Arts Decoratifs, also in the Louvre, is featuring an excellent show of Indian textiles, including fabrics woven with 22-karat gold thread.

And in February, the lively Pompidou Center will feature "Vienna 1880-1938," a look at the Viennese influence on European life, including art, architecture, furnishings and music.

Transatlantic air fares drop substantially in the off-season, making Europe a better buy for travelers on a tight budget. The major carriers -- Pan Am, TWA and British Airways -- are currently advertising a $438 round-trip fare between Washington and London (21-day advance purchase; available through the winter, except at Christmas). A comparable summer fare was $649.

But even lower fall/winter fares are possible, especially for travelers with flexibility.

And though the dollar has slipped in buying power abroad, Christmas gift buyers can still find bargains. A designer silk tie, selling for about $45 in the United States, is "under $30" in Paris, says Robert W. Bloch, spokesman for Galeries Lafayette, one of the city's largest specialty stores. Famous-name crystal, such as Baccarat, sells for about half the U.S. price -- but shoppers must remember to obtain a sales tax refund. (You must fill out a refund claim at the store, and usually the refund can be obtained at a special booth for that purpose at the Paris airport. A sales clerk can instruct you in the proper procedure.)

Galeries Lafayette and Printemps, another major Paris store, have put together pre-Christmas shopping packages for Americans. And Harrods, the famous London emporium, is preparing for a flood of transatlantic shoppers at its big after-Christmas sale from Jan. 8 through Feb. 1.

Here's a look at some off-season air fares and special shopping and theater packages to Europe:

*Air fares: In this day of frequently fluctuating air fares, trying to find the cheapest seat to Europe can be a challenge. But the sampling of fares that follows suggests what a transatlantic traveler should expect to pay.

Generally, the low-fare season runs from now through the end of March or into April. The exception is the busy holiday period from mid-December (Dec. 12 for several airlines) to Christmas Day, during which many fares jump back to about what they were during the summer.

As cited above, a round-trip, 21-day advance-purchase ticket (non-holiday) between Washington and London is being advertised for $438 by Pan Am, British Airways and TWA. That is for weekday departures, Monday through Thursday. The weekend fare, Friday through Sunday, is $488.

But there are variations on this price:

TWA promises a $25 discount on its Europe-bound fares if you buy the ticket from a travel agent. In addition, two people flying together are entitled to a week's use of a Hertz economy rental car at no extra charge -- impractical in London but handy should they want to travel into the countryside.

With its $438 round trip between Washington and London, British Airways offers a choice of four discounts for purchasing through a travel agent: a free theater ticket; use of a rental car for two days; a $25 discount on a tour; or a $25 discount on the air fare.

World Airways, flying from Baltimore/Washington International, is offering a $398 round-trip ticket to London. But the fare carries heavy restrictions. You must make a reservation 30 days in advance; pay on the day you reserve; stay abroad no more than 21 days; and there is a $100 cancellation penalty. World's round-trip fare to Frankfurt is also $398. The restrictions are the same except there is no advance-purchase requirement.

Surprisingly, People Express, which made its name with budget fares, is charging about the same for a London ticket as the major carriers, but with an odd switch. People Express' lowest fare, $436 from Washington/Dulles International, is on the weekend. Its lowest weekday fare, $456 from Dulles, is actually higher than that of the major carriers.

One advantage to the airline's fare, however, is that it is free of the advance-purchase requirement and other restrictions.

Virgin Atlantic Airways, another budget airline, is promoting a "Latesaver Fare" to London of $169 (or $338 round trip). But departure is from Newark International Airport in New Jersey, so you have to get there first -- an additional cost. A seat can be reserved only two days before departure.

For travel to Paris, TWA is advertising a $536 round-trip fare (21-day advance purchase, Monday through Thursday). A comparable Washington/Paris round trip last summer was about $800. TWA's offer of a $25 discount for buying from a travel agent, and a rental car free for a week for two passengers flying together, also applies to its Paris flights.

Two of the lowest fares to Europe will take you to Brussels and Luxembourg City. Both are interesting to visit, and they are convenient to other European cities. Luxembourg is about a four-hour train ride from Paris.

People Express is selling an unrestricted round-trip ticket between Dulles and Brussels for $336, which is only $19 more than its cheapest advertised fare to San Francisco and Los Angeles.

Icelandair has a one-way standby fare of $168 ($336 round trip) between BWI and Luxembourg. For this fare, however, you can reserve for a flight only on the day you want to go. If the plane is full, you will have to try again another day. From Luxembourg, the airline offers free or reduced-price bus and train transportation to other European cities.

There is, of course, no guarantee these fares will stand throughout the winter. Anyone considering a trip abroad should watch for special promotional fares advertised in newspapers and check with a good travel agent.

*Special tour packages: Galeries Lafayette and Printemps in Paris have each linked with an international airline -- Pan Am and Air France, respectively -- to offer one-week hotel and sightseeing packages for Christmas shoppers.

Americans still will find "fantastic" shopping in Paris, assures Nora Brossard of the French Government Tourist Office in New York, even though the dollar doesn't buy as much as it did earlier this year.

For $299 per person (double occupancy), the Pan Am/Galeries Lafayette package provides six nights in your choice of three "first-class" hotels with breakfast and a pass on the Paris Metro. For $199 per person, the offer is three nights in a Paris hotel, three nights in Nice on the Riveria (breakfasts included) and seven days' use of a rental car. The airline suggests you drive to Nice and fly home from there.

The Air France/Printemps package provides six nights at the Hotel Concorde-Lafayette for $199 per person (double occupancy). Air France passengers also get a 10 percent discount on purchases at Printemps and a complimentary Seine cruise. Air fare is extra for these shopping packages.

A new "Guide to Paris Shopping" is available free from the French Government Tourist Office, 610 Fifth Ave., New York, N.Y. 10020, (212) 757-1125.

Among the theater packages organized by British Airways is one that guarantees "the finest seats" for the two hit musicals "Cats" and "Starlight Express." Your trip must be arranged, however, so that you can see both in the same week -- "Cats" on Tuesday evening and "Starlight Express" on Friday evening. The package also includes six nights' accommodations and two pre-theater dinners. It ranges from $292 to $451 per person (double occupancy), depending on the quality of hotels chosen. Air fare is extra.

On any trip abroad, it's good to check in advance on national holidays, when museums may be closed, public transportation limited and few restaurants open. This is particularly true in winter, when most sightseeing is indoors. Christmas, for example, can be a very quiet time in some European cities. Activity in London all but ceases for three days, from Dec. 24 through Dec. 26.