The Hotel Association of Washington, calling union contract demands for a one-year, 20 percent raise for 6,000 hotel workers unrealistic, broke off negotiations with the labor group yesterday and said it has asked for help from a U.S. mediator to avoid a threatened strike Sept. 16.

Both the association and the union, the Hotel & Restaurant Employees local 25, AFL-CIO, said a strike still could be avoided, but neither was optimistic.

A strike by union desk clerks, waiters and waitresses, maids, janitors and bellmen would disrupt most of the city's major hotels and come just as the hotel industry begins its peak business period after a summertime lull.

The last strike by hotel workers was in 1946 and lasted 21 days.

Union officials, who crowded into a press conference called yesterday by the association at the Washington Hilton, sharply criticized the hotel owners for appealing to the Federal Mediation and Conciliation Service without first offering any contract proposals.

Union president Ronald Richardson said the local would meet with the mediator today but would not negotiate until the association responds with a counter proposal to the union's opening proposal for a pay increase that would raise the average hotel worker's hourly pay 93 cents, from $4.65 to $5.68.

"We're not going to bargain against ourselves," Richardson said.

Allen G. Siegel, the association's labor attorney and spokesman on the negotiations, said the hotel group, which has 53 member hotels and represents 23 of them in labor negotiations, said "our intention is to negotiate a contract . . . without a labor dispute. I don't know yet that the optimism I have . . . is justified."

He charged that the union demands for pay increases, improved health benefits, payments for employes laid off during slack hotel periods and other economic issues would amount to about $100 per employe per week and force unionized hotels to raise daily rates by $25 a room.

Siegel said the negotiations "are in a deep funk."

He acknowledged that the association has not prepared a counter proposal for the union or the mediator and has met with the union only twice. "We have met, but we have not in fact negotiated," Siegel said, adding that the union proposals are so high "we do not have the ability to respond to them."

Mayor Marion Barry is expected to take an interest in the negotiations, not only because of the damage a strike would do to the city's convention and tourism economy, but also because he counts hotel owners among his potential financial backers for his expected reelection bid, according to city and business officials.

The association and union have found it difficult to get together partially because the two sides are suspicious of each other's motives, according to observers familiar with the negotiations.

Some hotel owners feel Richardson, the union president, has been trying to increase his own stature within organized labor by generating support for a strike among his membership.

Other observers said the hotel association is poorly organized, that members from chain hotels are at odds with independent owners over negotiating goals.

The association also may believe the political and economic climate is right to take a hard stand against the union, pointing to President Reagan's handling of the air traffic controllers strike, according to observers.

The hotel owners also are concerned about increased business pressure from largely nonunion suburban hotels that have been attracting more of the area's convention-oriented business.

Siegel said the association's hotels have signed a lockout agrement to bar all of the union's workers if a strike is called against any of the member hotels. Hotels that do not abide by the agreement will be fined $25 a day per room if they allow union employes to work, Siegel said.