The 52 freed American hostages, already checked out by their doctors and reunited with their families, will arrive in Washington today for an official "welcome home" ceremony and public celebration before they finally return to their hometowns and attempt to live normal lives again.

Hundreds of thousands of people are expected to line a 12-mile motorcade route in the early afternoon between Andrews Air Force Base and the White House, where the former hostages will be met by President Reagan and the entire United States Congress.

The ubiquitous yellow ribbons that have become a symbol for the long-awaited return of the hostages, will adorn everything along the motorcade route from ponytails to liquor stores to the District Building. On the Mall tonight there will be a fireworks extravaganza grander than anything done here on the Fourth of July. Local police say they expect the crowds all day to be joyous and easy to handle.

"This promises to have one of the largest crowds for a parade or motorcade that we have ever seen in Washington," said George Berklacy, a spokesman for the National Park Service.

The 52 former hostages and their relatives, who have spent the past two days in the relative privacy of West Point, are expected to arrive at Andrews at approximately 11:55 a.m. There they will be greeted by Vice President George Bush, Secretary of State Alexander M. Haig, Defense Secretary Casper Weinberger and congressional leaders.

About an hour later, when they depart from Andrews in 12 new Metro buses, they will be encountered by banners and flags and ribbons and flowers everywhere along a motorcade route that will take them through Prince George's County on Suitland Parkway and then to the White House along the same route that President Reagan followed on his inauguration day one week ago.

Along the way the returnees and their families will be wept over and cheered. At Andrews, which will be closed to the public, the Iron Workers Union will unfurl "the world's largest flag." Prince George's and District fire departments plan to form arches with ladder trucks through which the motorcade will pass. A plywood Statue of Liberty, carved with a jigsaw over the weekend by a house painter named Bob Walker, stands atop a "Welcome Home America" float alongside the parade route. Fifty-two lasers will again strafe the sky on the Mall tonight. And the National Christmas Tree will be lit for one last night.

Louise Lantz, a Prince George's County woman whose ribbon-bedecked front yard faces the motorcade route, has chosen to delay the funeral of her mother-in-law so she can "whoop and holler" when the former hostages pass.

Metropolitan and U.S. Park Police said yesterday they have no way accurately to estimate how many people will show up for the 90-minute motorcade. The federal government and local schools are not planning to close down for the event. But officials said they would not be surprised to see one of the largest crowds in the history of Washington.

"This has been such a gut thing for the people of this country that we expect a great crowd," said Alan Grip, a spokesman for District Mayor Marion Barry.

According to Park Police, the best places in the District to watch the motorcade, which will be moving at about 15 miles an hour, will be on Third Street in front of the Capitol between Independence and Constitution, on the corners of Pennsylvania Avenue and 14th Street and at Western Plaza near the District Building. In Prince George's County, the best place to view the motorcade will be from an access road that parallels Suitland Parkway between Suitland Road and Silver Hill Road.

The State Department insisted on a motorcade in Washington, rather than a more elaborate parade, because of concern over the psychological conditions of some of the former hostages. "Our principal concern is for the health and well-being of the individuals," State Department spokesman William Dyess said in a briefing yesterday. "Some may not yet be up to such a strenuous schedule."

(Officials in New York yesterday ignored the Reagan administration's wishes and planned a ticker tape parade for the 52 on Friday, inviting any of the former hostages who would come.)

After a two-hour reception and meeting with Reagan and members of Congress at the White House today, the returnees will be driven to the Crystal City Marriott for a one-night stay. On Wednesday they depart for their home towns and what may be months of decompression from their 444 days of captivity and their instant celebrity status.

The homecoming of the former hostages will be celebrated tonight on the Mall with a $25,000 fireworks display that pyrotechnician George Zambelli of New Castle, Pa., says will be larger than last summer's Fourth of July show.

The program, which begins at 6:45 near the Washington Monument, has been choreographed to begin with the firing of 52 exploding rockets, each carrying a yellow ribbon. Eight additional rockets will also be fired carrying red-white-and-blue ribbons commemorating the eight Americans who died last spring in the abortive rescue mission in Iran.

Following the rockets, a 50-by-50-foot ground display suspended on scaffolding will be ignited to display the words "Welcome Home." The fireworks show will be accompanied by patriotic music broadcast from loud speakers on the Mall and on radio station WGMS. Zambelli and the radio station are paying for the fireworks.

Mayor Barry will today issue a proclamation "honoring the 52 Americans" for their "selfless and personal sacrifice during this ordeal." The proclamation calls on all District of Columbia residents to pay tribute to the 52 who "have sacrificed so much for our country."

Metropolitan police said yesterday that traffic barricades will not be erected along the motorcade route, as was done for the inaugural parade. Traffic will be interrupted only to allow the motorcade to pass.

Suitland Parkway, which was being cleaned of trash yesterday by six Park Service maintenance crews, will remain open throughout the day. Motorists will be ticketed, however, for parking on the shoulders of the parkway during the motorcade, according to the Park Service.

With heavy crowds expected in the afternoon near the Capitol, Metro yesterday advised commuters to buy their farecards in the morning to avoid long delays. Metro will be running additional subway trains in the afternoon.