Five thousand Washington Post employees and their families streamed onto the company's tentbedecked parking area yesterday to eat, dance and mingle with friends in celebration of the newspaper's 100th anniversary.

Twice as many people as originally had been expected showed up to enjoy free food and entertainment and to tour the Post building on a sunny, breezy afternoon. The Post employs about 2,500.

Square dancing, bluegrass, a jazz band, a Carib-bean steel ban and a rock group entertained and guests lined up for hamburgers, hot dogs, potato salad and cake. A hundred kegs of beer were consumed.

Children flocked to various rides and pinball machines, to cotton candy and snow cones and to Winkie the clown, who tied ballons into animal shapes and dusted children with cofettti.

"This is the greatest day of my life," one child, Jennifer unes, told her mother, "because everything is free and I can have as much as I want." An hour later the child was exhausted and ready for a nap.

Washington Post publisher Katherine Graham, and her son Donald, executive vice president and general manager of The Post stood at the entrance of the parking area and greeted employees through most of the seven-hour affair.

"To see all these people having such a good time, it's togetherness, it's just, well, right," Mrs. Graham said. "It's very important for the family and children of employees to see the building and feel part of it, and for people who work in the different departments to meet their fellow workers and their families."

Donald Graham greeted virtually every employee by name.

"It's much better than I'd hoped for," Mrs. Graham said. "Even the sophisticates from the newsroom came."

The 100th anniversary celebration was without speeches or any formal ceremony. "Nobody wants that kind of thing - they just want to have a good time," Mrs. Graham said. Late in the afternoon she did cut a birthday cake and then went square dancing.

The Post was founded by Stilson Hutchins in 1877. The first issue was a modest four pages and sold for 3 cents. The paper was put up for auction in 1933, and was purchased by Eugene Meyer, a California-born banker with a notable record of service in government. His daughter Katharine Graham took over in 1963.

"This is not like a typical employee-employer organization; it's always been much more like a family thing," remarked Casper Miles, a composing room employee for 22 years and one of the many retirees who came to the party.

People were reluctant to leave even at the 6 p.m. closing and after several farewell numbers by the band. As he left, the son of one employee said goodbye to a friend he had met at the pinball machines during the centennial celebration. "See you next year," he said.