When the massive Washington Hilton Hotel opened in March 1955, just 15 months after the assassination of President Kennedy, little attention was made of a specially designed passage built to allow visiting presidents to be whisked inside the hotel, away from crowds and the security risks they cause.

The private corridor, which was approved by the Secret Service, according to an architect whose firm designed the building, is bathed in indirect lighting and decorated with deep-pile red-and-blue carpeting and snow-white brocade wallpaper. The passage leads from the hotel's main ballroom to unmarked, steel-gray double doors that open onto the T Street side of the Hilton.

This is the inconspicuous "VIP" entrance to the hotel -- and the hideaway corridor from which President Reagan had just emerged when he was shot in Monday's assassination attempt. Questions were being raised yesterday about why the president's limousine was not parked directly in front of the private double door. The hotel access road is only five steps from the doors, but Reagan's limousine was parked at least several more steps away.

Yesterday, the hotel's side entrance was swamped with tourists and the curious, many competing with news cameramen for the best angle to take their pocket camera snapshots of the spot where Reagan was shot.

Hotel officials declined to discuss the private hallway that every president since Lyndon Johnson has used. But hotel officials reluctantly admitted its existence.

The exterior doors today appear to lead to a work area of the Hilton. But when the hotel first opened, a sign over the doors read: "Presidential Entrance." White House officials banned the sign for security reasons before the hotel opened, according to a brief press report in 1965.

About 7 feet wide and 8 1/2 feet high, the richly appointed walkway leads for almost 100 yards around curves and down a stairway. It ends about 30 steps from the backstage area of the Hilton's huge International Ballroom, which can hold more than 5,000 persons. Presidents and other dignitaries can slip in and out of the hotel on the ballroom's Terrace Level without wading through exuberant and potentially dangerous crowds.

At least two private elevators that must be operated with keys are inside the secret passageway. Doors also lead from within the corridor to a private foyer and to other locked rooms. A few of the doors, all locked, were still marked with faded security tape that had been stripped across them on more than a dozen occasions. Printed on the security tape is a message warning passersby not to open the doors without contacting the Secret Service.

At least two sets of telephones are in the hallway.

The secret passage lies behind another unmarked door at the ballroom arena that separates it from the public ballroom and a "President's walk," a long corridor that wraps around part of the ballroom and is hung with reproduction pictures of all the presidents and their wives. The Reagans' pictures have not yet have added.

The plush tunnel also is set off from adjacent utilitarian hallways for hotel employes to get to kitchens, a parking garage and back stairways. Those work areas, lined with unpainted concrete blocks, are littered with unused tables, empty wine bottles and water pitchers, and several rat poison containers.

New York architect David Dann, whose firm, William B. Tabler, Inc., designed the hotel, said yesterday that Hilton officials were thinking of not only the president's security but also his dignity when they designed the passage.

He recalled that when President Franklin Roosevelt visited the Statler Hilton on 16th Street (now the Capitol Hilton), which was built in 1942, he was forced for security reasons to to leave through the hotel's kitchen.

"They didn't want to do that again," said Tabler.