As the last flowers and decorations went into place and police began a huge security operation, thousands of Britons and foreign visitors today thronged the procession route for Wednesday's wedding of Prince Charles and Lady Diana Spencer in what is becoming a week-long national celebration.

Following a surprising turnout of about 50,000 spectators for yesterday's dress rehearsal of the royal carriage procession between Buckingham Palace and St. Paul's Cathedral, large crowds were out today to walk along the colorfully decorated two miles through the historic heart of London.

Some people began camping out in choice viewing positions for Wednesday's pageantry, others admired the thousands of red geraniums in front of Buckingham Palace, the hanging baskets of blue, pink and white petunias, verbena and phlox, and the miles of bunting and profusion of flags, heraldic emblems and signs congratulating the royal couple. Many stopped to gawk quizzically at the open-air studios on the edge of Green Park opposite the palace, from where ABC and NBC were broadcasting their early morning television shows at lunchtime here.

The crowds around St. Paul's swelled tonight and traffic in central London ground to a halt when Charles and Diana showed up for a final 30-minute rehearsal of the wedding ceremony. Diana was sneaked in through an underground garage entrance beneath the vast cathedral, but she and Charles later left hand-in-hand by the great west door and down the broad steps where she will arrive with her father to the fanfare of trumpets on Wednesday.

The couple later went to the palace for a dinner with close relatives and friends, followed by a grand ball for about 1,500 guests, including the royal family's European relations, members of the palace staff, and many other British, European and American friends of the bride and groom, Queen Elizabeth II and Prince Philip, and their families. It is to be the last time Charles sees Diana until the wedding ceremony begins.

Crowd control barriers and directional signs were already up along the procession route and police began detailed inspections of the barriers, stands for spectators and reporters, manholes and sewers and the cathdral itself for explosives or signs of suspicious activity.

After being toured by record numbers of visitors over the weekend, St. Paul's was closed until the wedding except for Nancy Reagan's tour of it today, the last-minute preparations being made by workmen and television technicians, and tonight's rehearsal.

Police officials described how 4,000 officers, some of them armed, augmented by nearly 3,000 honor guard troops, rooftop antisniper patrols and an antiterrorist commando unit will line the procession route on Wednesday and take other steps to protect the royal family and visiting monarchs and heads of state. Closed-circuit television cameras and a police helicopter flying overhead will relay pictures of the entire route to a command center where officers will be able to zoom in on suspicious faces or threatening behavior.

The tight security will be a reminder of the backdrop of upheaval in British cities and British-ruled Northern Ireland that lies behind the pomp and escapist euphoria of the royal wedding. Rioting occurred again over the weekend in northern England in inncer city Liverpool and the resort Lake District town of Keswick. Parliament today debated the rapidly rising unemployment rate, which is believed to be one of the root causes of the rioting. In Northern Ireland, two of the eight current Irish nationalist hunger strikers remained near death and the deteriorating condition of a third has created concern.

Such unpleasant realities appeared to be quite remote from the multitudes strolling up and down the royal wedding route on a pleasantly warm midsummer day. Many, like Norman Meade, a civilian worker for the Royal Air Force at High Wycombe, 30 miles outside London, had just come for the day to show his children where history will be made on their television screens Wednesday.

"It will be so crowded here that day, and I'll be working," said Mead, as his wife photographed their son and daughter against a background of red geraniums across from Buckingham Palace. "I figured this was the chance for the children to see everything just as it will be on the big day."

Averil Harrison, a private school-mistress from southwestern England, and her 18-year-old daughter, Rosemary, have decided to stay, however. They became the first to camp out along the route on Sunday afternoon when they unrolled their sleeping bags midway along the Mall a few hundred yards from the palace. The wedding procession will pass within a few feet from them when the carriages roll down the red-paved royal road past St. James' Park toward Trafalgar Square Wednesday morning.

"I'm glad we came early," said Harrison, who brought enough food for the whole week. "We are here for the atmosphere. You see a lot more on the television, but you miss actually being involved."

By noon today, the Harrisons had given more than 30 interviews to the world's press and been taped for television by British and American networks. More than a thousand reporters from around the globe have been accredited for the wedding, in addition to the armies of technicians and backup personnel.

On side-by-side raised platforms a story or two above the grass of Green Park across from the Buckingham Palace, rival NBC and ABC crews broadcast the Today and Good Morning America shows to the United States today as passing British promenaders puzzled over who these Americans were under the bright television lights.

"Terry Wogan, I'll bet," declared one elderly gentlmen who decided it must be something hosted by Britain's most popular television and radio personality. Actually, he was watching Tom Brokaw and Jane Pauley interview Tina Brown, editor of the Tatler, a glossy London gossip magazine.

ABC's Barbara Walters had to go to last night's star-studded dinner for Nancy Reagan at the residence of new U.S. Ambassador John J. Louis Jr. to be recognized by the other imported U.S. celebrities. She said that when she was introduced to Prince Charles at a Buckingham Palace garden party two weeks ago, he first asked what radio network she worked for and then wished her good luck on CBS' wedding coverage.

In addition to the outdoor studios near Buckingham Palace, both ABC and CBS have built booths for their wedding day anchors outside St. Paul's.

British television and radio also are saturated with prewedding shows and will begin their wedding day coverage unusually early, with preview programs starting at 6 and 7 a.m., long before the 11 a.m. service. The newspapers are filled with special wedding sections that will continue through next weekend.

Britain's largest-selling tabloid, Rupert Murdoch's racy Sun, has renamed itself the "Royal Sun" for this week and christened its bare-beasted pinup girls "Page Three Princesses." The Sun contained no less than 10 pages of wedding coverage today, including a circulation-building contest offering as first prize a replica of the four-poster bed Charles and Diana are expected to begin their honeymoon in.

At the other end of the market, the staid Times, now also owned by Murdoch, is publishing its first color magazine as a wedding souvenir on Tuesday. The equally up-market Guardian, the most irreverent about the royal family, today published a typically syrupy essay on the virtues of romance by best-selling novelist Barbara Cartland, Lady Diana's stepgrandmother and an account of a recent, modest Register Office wedding for two welfare recipients in Newcastle, which has been hard hit by Britain's deep recession.

The numbers of foreign tourists here for the wedding are fewer than originally expected, although travel agencies and hotels report a recent increase in bookings by Americans for later in the summer and autumn. The vast majority of people expected along the procession route Wednesday will be British, both Londoners and provincial residents.

Heading the foreign visitors will be monarchs and heads of state from 50 countries, including the kings and queens of Belgium, Norway, Denmark, Sweden, the Netherlands, Tonga and Lesotho and the former kings of Greece, Bulgaria and Romania. The just-released list of dignitaries also includes princes and princesses of Japan, Monaco, Liechtenstein, Jordan, Thailand, Nepal and Swaziland, plus the grand duke and duchess of Luxembourg.

Many of the statemen will be moving about London and traveling to St. Paul's Wednesday in the increasingly numerous Rolls Royces appearing on London streets, belying the country's economic crisis. These dignitaries have not commandeered all the Rolls, however, ABC television has found one to take its superstars around London in regal style.