With Washington's busy spring tourist season approaching, Gold Line-Gray Line, the city's biggest tour-bus operator, has been stalled by a strike of its 150 union drivers and mechanics protesting a company plan they contend could result in 20 percent pay cuts and would penalize workers calling in sick.

The company, which carries more than 1 million charter customers and sightseers each year, temporarily canceled tourist runs and charters yesterday as its fleet of some 90 buses was idled at company headquarters in Tuxedo in Prince George's County where the Amalgamated Transit Union set up picket lines.

Gold Line Inc., which also operates four commuter runs between Washington and Fairfax City and Sterling in Virginia, and Charles and St. Mary's counties in Maryland, has arranged for other companies to temporarily take over the two Virginia lines, but has suspended the two Maryland commuter runs, according to industry officials.

Gold Line, which operates tourist buses serving major downtown hotels under the Gray Line name, has been financially strapped since deregulation of the transportation industry has allowed dozens of smaller competitors to carve up portions of the lucrative Washington tourist and charter business, and the company has sought what the union calls severe cuts in pay and benefits.

Gold Line officials initially declined comment on the strike yesterday, and did not return repeated telephone calls. The firm's contract with Local 1098 of the ATU expired Jan. 15, but the union extended it during negotiations that had continued under supervision of the Federal Mediation and Conciliation Service. But those talks broke off and no new talks are scheduled. Union members said it could be a long strike.

A key issue in the walkout, which began Monday, is a company proposal to discontinue a "piece work" system that guaranteed drivers 3.5 or 4 hours' pay for each local assignment, according to union president Ed Strait. Under the Gold Line proposal, drivers, who often work irregular hours, would be paid only for actual driving time. Drivers earn $7.37 to $8.37 hourly.

"I would lose about $5,000," said Sarah Hanna, 29, a four-year driver. "If you don't have the four-hour rule, you have these spaces of time during the day where you are not paid." Hanna said she earned $23,000 last year by often working 10-hour days and six-day weeks because drivers are not paid overtime.

"They presented us with a contract offer they know we could not accept to force this strike . . . . They want to force the union out," said Hanna, who walked the picket line at company headquarters.

The union had previously agreed to a pay freeze, and is willing to make concessions, Strait said, but union members "refuse to accept . . . sweatshop rules." Drivers are not paid for sick leave, but the new rules proposed by Gold Line would penalize drivers on the second occasion of calling in sick by assigning them to the bottom of the priority list for driving jobs, Strait said. The firm would also be able to fire drivers who receive two letters of complaint from customers, which Strait called "ridiculous, because drivers handle thousands of people a year, and personalities do clash."