Not since the early 19th century, when Alexandria's cobblestone streets rang with the sound of horseshoes, and tobacco merchants lounged along the bustling waterfront, has a larger fleet sailed into the city's harbor.

They came singly yesterday. Eighteen tall ships, majestic brigantines and tidy schooners, their sails snapping, crowded the harbor as the city prepared to celebrate the fourth annual Red Cross Waterfront Festival today and Sunday.

The festival's guest of honor, the Colombian naval vessel Gloria, filled with eager cadets and loaded with artifacts, arrived Thursday. Visitors will be welcome throughout the weekend to explore the 255-foot ship, docked at the Robinson Terminal.

"Oh, Mommy, is this President Reagan's boat?" asked Elizabeth Huffman, 7, as she rubbed sunburned hands along the brilliantly polished brass rails and wandered below deck to the "Museum of Gold," the Gloria's ornate collection of archaeological treasures from Colombia.

Volunteers working with the Red Cross and the Alexandria Tourist Council, the organizations sponsoring the festival, were working overtime yesterday, trying to arrange the logistics of an event that is expected to draw more than 100,000 visitors this weekend.

"I spent several days last week worrying whether all the underwear was labeled properly," said Diane Bechtol of the Tourist Council, talking about the difficulty of getting laundry done for 150 Colombian sailors. "And do you have any idea what it's like to order hot dogs for 80,000 people?"

The laundry problem, potentially severe for 150 Spanish-speaking sailors who normally rely on coin-operated machines in foreign ports, was solved easily enough when Alexandria Hospital heard their distress signals and offered to take on the chore.

As food vendors delivered immense amounts of everything from hot dogs to "nautical noodles" to hundreds of canisters of Coca-Cola along the waterfront, a huge truck from the hospital collected 1,800 pounds of clearly labeled Colombian laundry and hauled it off to the medical facility's extra large washing machines.

"It seems like a minor matter, but for the sailors it's a big problem," said Dave Norcross, the hospital's public relations director and an avid sailor himself. "We usually handle more than three tons a day, so big loads don't scare us."

Today's festivities will begin with an opening procession at 11 a.m. The parade, which will begin at Market Square, features the Redskins band and the Redskinettes along with local radio personalities Frank Hardin and Jackson Weaver and the ships' crews.

There also will be strength and fitness tests available, boat tours and plenty of jazz.

Visitors willing to spend a dollar will have the chance to become part of a historic enterprise by eating part of what is being billed as the longest pizza ever made. Hungry's Pizza of Alexandria will try to break the world record of 375 feet set last year in Tennessee.

If things go well today, Ted Galanos, the man with all the dough, will try to break the new record on Sunday. Galanos said he is planning to use more than 800 pounds of cheese and 50 gallons of sauce in his effort to break into the record books with his mammoth pizza.

But it is the ships that are expected to draw the largest number of fans.

"I can't believe this thing," said Lt. Cmdr. Ernest Evans, a supply officer who is helping the Gloria with supplies and repairs. "I have been in the Navy for more than 13 years and I have never been on a sailing ship before. This thing is a real winner."