Washington Gas Light Co. was hit with its first strike in 25 years yesterday, as 1,250 service workers walked off their jobs. Another 500 office workers also threatened to strike in disputes over wages, job security and what union officials called excessive "forced overtime."

The utility said its 590,000 customers -- including more than 400,000 residential users of natural gas -- should feel no immediate impact from the strike by the International Union of Gas Workers, an independent union that had never struck in 20 years at the company.

The company assigned some 900 managerial staff members to 12-hour shifts to answer emergencies and disruptions in service, but it postponed routine calls and new installations.

"We struck because they wouldn't give us security on wages or on job security," said Robert Pellegrino, the union president. He said union members voted by a 9-to-1 margin to authorize a strike, primarily because the company is seeking to eliminate cost-of-living adjustments and is reducing its work force.

"It is a very regrettable situation," Washington Gas Chairman Donald J. Heim said in a statement. "We've made a very fair and good offer to the unions. And with one exception, 25 years ago, we've never had a strike. Normally we're a tight-knit group."

The company offered a three-year contract with annual raises of 4 1/4 percent. Union members, who install, repair and maintain gas lines, earn an average of about $13.50 hourly; until now, they have been partially protected from inflation by a cost-of-living (COLA) clause. The company said its offer also included improvements in medical and dental benefits.

Company and union officials planned to meet today at the Federal Mediation and Conciliation Service in an attempt to resolve the strike. The union contract expired June 1 but had been extended until 12:01 a.m. yesterday.

Members of a second union, Local 2 of the Office and Professional Employees International, which represents clerical and office workers, also authorized a strike, but they had to await approval of national union officers before striking, officials said.

Washington Gas enjoyed increased profits in 1984 and 1985 while embarking on a cost-cutting program by reducing its work force. In 1982 the company announced a target of cutting staff by 10 percent through attrition, but cuts have been more than 15 percent. The union said management has been reduced by only 2.7 percent during the same period.

"The unions are unhappy with this, but we believe it is a legitimate cost-containment program, and we have not laid anybody off," said WGL spokeswoman Cate Barnett.

Pellegrino said the cuts have resulted in increased overtime of as much as 50 percent by gas-service workers since the 1982 union contract began. Many workers like the time-and-a-half overtime pay, he said, "but not when you are working so much of it." Barnett said overtime averages only four hours a week per worker, "and we feel this is a reasonable level."

During the three-year contract term that just expired, WGL's top management received pay raises of more than 50 percent, while union members received 19 percent, Pellegrino said.