Suddenly, London traffic seems worse than ever, and finding an empty taxi is impossible. Crammed city sidewalks ring with the transatlantic twang that Britons find so distressing to the ear. Not a hotel room, theater ticket nor restaurant reservation is to be had for love or money, and the weather suddenly has turned from unseasonable chill to a breezy sunshine Londoners consider a heat wave.

The temperature is one of the few things wrong with London this week that the British are unable to attribute to the largest organized force of Americans to hit their island since World War II.

Each member of the army sports a red, white and blue name tag identifying its wearer as a member, spouse or friend of the American Bar Association, holding its annual convention here in the cradle of common law for the first time in 14 years.

Actually, this is only one "section" of the ABA convention. Lawyers in quest of a tax-deductible summer trip could also have gone to Washington, D.C., where some may have even had business. But while 7,000 chose the muggy heat along the Potomac, the legal force here arrived 20,000 strong.

If the invasion of "the lawyers" is impossible for a Londoner to ignore this week, the locals are not entirely unhappy with their pervasive presence. For every disgruntled local theatergoer, a dozen toilers in the tourist industry and service sector joyfully partake of the $40 million the ABA conventioneers are expected to spend in Britain.

In honor of the event, Harrods department store has moved its summer sale to coincide with the arrival of the nearly 10,000 lawyers, more than half of whom have arrived with family in tow. Selfridges, its slightly downscale competitor, is running a special ABA "welcome" advertisement in local newspapers ("a very warm British welcome to all our American friends . . . you'll find that we stock quality British-made goods as well as all the great British names . . . Royal Doulton or Wedgwood, Aquascutum or Jaeger, Dunhill or Burberry . . .")

The convention is the largest such gathering ever to book itself into the British capital. The ABA, with the help of the American Express travel service, reserved a total of 120 hotels for the six-day event, including most of those in the upper-class Mayfair district during a time of year when normal occupancy tends to exceed 90 percent.

With the exception of the opening and closing ceremonies, there are no central meetings. The convention is organized into dozens of overlapping panel discussions on subjects like "The Bill of Rights: Creation or Cremation?," "Reorganization of an Airline Under Chapter 11 of the Bankruptcy Code and Its Impact on Aircraft Financing Transactions" and "Cohabitation Without Formal Marriage."

The sessions are being held in conference rooms throughout the city, in widely scattered corporate offices and hotels. To carry participants through the near gridlock of London summer traffic, the organizers have hired 4,000 London taxis full time.

Some conference speakers are bigger draws than others -- Prime Minister Margaret Thatcher, U.S. Chief Justice Warren Burger and British Lord Chancellor Lord Hailsham of St. Marylebone, complete with his judicial wig and robe.

Some sessions have been more exciting than others. Former vice president Walter F. Mondale's opening presentation to a panel on terrorism was interrupted by a loud explosion that brought security men (attending various British and U.S. officials) out of their seats. When the explosion appeared to have come from a television camera light, the audible relief was dampened by the subsequent downpour from the automatic fire sprinklers in the ceiling.

Speaking over the spatter of falling water, Mondale noted that "once you're out of office, you find there is very little dignity in this world."

Attendance at many of the sessions, reportedly, has been light. One London barrister who spoke to a plenary session early in the week was surprised that his audience barely topped 100. "A lot of the Americans seemed just to rush in to sign their forms and leave again." The forms are necessary to prove attendance at business-related functions to comply with U.S. tax-deduction regulations.

Under a new interpretation of 1976 tax legislation, this is likely to be the last such deductible ABA convention, and those who view the trip as at least partially a vacation have every opportunity to take full advantage.

Among the "special entertainments" arranged for those off-hours between meetings are:

A "pub crawl" along the Thames River from Westminster to Greenwich ($10); a "rare showing of the latest in Europe's top fashion design" presented with dinner at Maxim's de Paris in London ($45); a Scottish Evening at the Caledonian Club; a tour of famous and picturesque gardens around London; visits to the Royal School of Needlework; and courses in Cordon Bleu cookery and Constance Spry flower arranging.

With tickets to current London stage hits such as "Cats" and "Starlight Express" arranged by top London agent Keith Prowse, virtually every performance of every show in town has been sold out to conventioneers.

Day trips have been arranged to Brighton, Stratford-on-Avon, Salisbury, Stonehenge, Bath, Portsmouth, Cambridge, Oxford University, Blenheim Palace, Canterbury and Boulogne. Once the conference is over, a side trip to Leningrad has been arranged for a limited number of participants.

And, for a select group of 600 ABA members, Queen Elizabeth II is giving a garden party.