Music director Mstislay Rostropovich yesterday joined 25 musicians of the National Symphony Orchestra on their picket line in front of the Kennedy Center.

It was an unprecedented gesture of solidarity from a conductor for orchestra players whose strike went into its second day with no indication of when it might end.

Tonight's concert, the season's opener and the week's remaining concerts tomorrow. Thursday and Friday were cancelled. Officers of the National Symphony Association held a press conference to explain the financial difficulties that have led to the strike.

During the picketing, U.S. Park Police warned orchestra members they were in violation of the terms of their permit requiring them to walk on the east side of the roadway in front of the Kennedy Center. The musicians continued their picketing directly in front of the center.

At one point when as many as a dozen policemen gathered at the picket line. Rostropovich, in exile from his native Russia, quipped, "In my country, I've never been in jail."

After the fourth warning, however, the police drove away leaving the musicians to continue walking their line.

Also cancelled was last night's special concert for the World Bank Association. Initially, the Guarneri players heard about the stike, they, too, withdrew. Finally, Metropolitan Opera soprano Roberta Peters and baritone Robert Merrill agreed to perform, it was announced.

A decision still pressing symphony officials is the matter of the orchestra's scheduled trip to Mexico for six concerts to be played between Oct. 3 and 8. In a phone call yesterday morning to Mexico City, the orchestra management was told that a decision would have to be reached by the end of the day Thursday.

At the press conference, outgoing association president David Lloyd Kreeger said that expected funding had not materialized that would help the association to cover a present deficit of nearly $1 million and a further anticipated deficit of nearly $2 1/2 million more by the end of the current season.

For that reason, Kreeger said the board asked the musicians for a one-year contract, even though less than three months earlier the board had been insisting on a three-year cpntract. "Within that year," Kreeger said, "it is our belief that the financial situation of the orchestra and the availability of the essential federal funding would become sufficiently clear for us then to negotiate a three year contract, which both sides feel would be more agreeable.

"We would like one million plus if our goals are met," he explained. "That is out of a collective sum for the consortium of private artistic groups in Washington, of which that would be our share."

Both Kreeger and president-elect Austin Kiplinger emphasized that the prospects for the desired federal assistance were good. "It is the timing that has thrown us into a situation we did not anticipate," Kreeger said. Kiplinger added "In the meantime we are trying to tap special sources, private and foundations, to carry as over this particular time."

Noting that the National Symphony stands second or third in the country in public fund-raising, Kreeger pointed out that Washington lacks a General Motors or other major industrial sources for contributions like those that have provided large endowments for the orchestras of Pittsburgh and Minnesota.

As the press conference was ending, a federal mediator phoned to ask if a meeting could be set up for this morning. The picketing musicians believe the association must offer either a longer contract or better financial terms for a one-year duration.