Chanting slogans and brandishing signs, about 75 people who clean the insides of several of Washington's major downtown office buildings held a rally last night to begin what they said is a strike against the buildings' cleaning contractors.

The gathering was part of what officials of the Service Employees International Union said is a two-year-old effort to organize the 6,000 people who clean commercial office buildings in Washington. Union officials say most of the workers make $4.75 an hour, the minimum wage in the District.

The demonstration, which included chants in English and Spanish, began at 6 p.m. at Connecticut Avenue and L Street NW, outside Washington Square, where union officials said they are striking against United States Service Industries Inc., the cleaning contractor.

They said the rally was intended to kick off a strike at about a dozen downtown buildings, most of them on or near Connecticut Avenue NW, where they said about 130 people work for cleaning companies.

Jay Hessey, an organizer for Local 525 of the Service Employees, said about 90 percent of the workers had signed cards indicating that they want to join the union.

But he asserted that contractors at some of the buildings have been warning employees that the cleaning contracts might be terminated if the employees unionize.

Joel S. Felrice, treasurer of United States Service Industries, denied that employees were threatened about possible union activity. "We don't do that," he said.

Felrice said cleaning employees in Metropolitan Square receive more than the minimum wage. He said there are "various wages," depending on such factors as experience and tenure.

An official of Red Coats Inc., a maintenance company that cleans another Connecticut Avenue building where the union demonstrated briefly, also denied that employees have been threatened about possible union activity.

"Our people come to work because they want to," he said.

Officials of the international union, which they describe as the fastest-growing in the AFL-CIO, said about 700 janitors at commercial buildings in Washington have joined the union, with 500 working under union contracts.