Unions representing 15,500 District of Columbia government employes broke off contract talks with city negotiators yesterday, paving the way for a mediator to join the labor negotiations.

The union action came a day after the city reached a tentative agreement on a new contract with the police officers union, providing a 3 percent annual pay raise for the next three years.

Yesterday morning, Mark H. Levitt, city labor relations director, offered similar terms to the six unions representing the District's white-collar and blue-collar employes. But the unions, which have asked for 5 1/2 percent annual raises, called the offer unacceptable, and declared that an impasse had been reached after seven weeks of negotiations.

All the city's labor contracts expired Sept. 30. City law prohibits public employes from striking, but sets up an elaborate process of mediation, followed by binding arbitration, if agreements cannot be reached.

"We've gone as far as we can go at this stage of the negotiations," said Donald Wasserman, director of collective bargaining for the American Federation of State, County and Municipal Employees (AFSCME), one of the largest unions involved in the complex negotiations.

"For productive negotiations at this point we need the services of a third party," Wasserman said. "We're still very hopeful that a voluntary agreement can be reached . . . . But we want to start the clock going if we need arbitration. We don't want to be in a position six months from now where there is no settlement."

Levitt said he told the unions that the city wants to establish a pattern in its wage settlements, similar to the contract reached with the police officers, but he added: "We told them there's plenty of room to move."

"We think an impasse does not exist," Levitt said. "But if the unions need a mediator to help them put their act together, we have no trouble with that."

Both Wasserman and Levitt said the city and the unions would meet soon to choose a mediator. If they cannot agree on one, a mediator would be selected by the city's Public Employees Relations Board.

The current contract talks are the third round of negotiations between the city and its labor unions since the District separated its personnel system from the federal government in 1980. In the previous talks, agreement was reached on all major contracts, except for the police, without either a mediator or an arbitrator being involved.

Levitt said the District's 11,000 unionized white-collar employes have an average annual salary of $20,387, with pay scales ranging from $13,000 to $44,000 a year. He said average pay for the 4,500 blue-collar employes, including laborers, sanitation workers and tradesmen, is $21,348. Their pay scales range from $13,000 to $38,000 a year.

The District pays more than the federal government in virtually every job category, Levitt said, except for secretaries, who received special pay raises this month because of the shortage of clerical employes in the Washington area.

After the city contract talks broke off yesterday morning, about 70 members of the union negotiating teams staged a one-hour demonstration outside the District Building.

Separate negotiations are continuing with D.C. government nurses and firefighters. The school board is conducting negotiations with teachers.