The National Symphony Orchestra showed up at the Kennedy Center last night in white tie and tails and evening gowns.But it wasn't to perform. They were there to walk a picket line in the third day of their strike.

Music director Mstislav Rostropovich joined what appeared to be most of the 104 members of the orchestra manning the line at 7 p.m. Wives, husbands and children of the players swelled the crowd outside the Center.

Last night was to have been the NSO's season-opener, but that concert and those scheduled for tonight, tomorrow and Friday have been canceled because of the strike.

Representatives of the players and the National Symphony Association are scheduled to hold a negotiating session at 11 a.m. today. It will be the first since the strike began at midnight Saturday.

No negotiations were held yesterday because lawyers for the musicians were not availabel.

Small groups of pickets walked the line in front of the center all day, but as the 8:30 hour approached at which their concerts usually begin, the musicians came together for a special show of solidarity. Instead of instruments, they carried picket signs.

In their concert garb, the players formed a classy-looking picket line, though a cool wind proved chilling to some of the women in their evening dress.

During their picketing Monday, the players had been warned to move to the other side of the roadway in front of the center. They didn't, and there were no further warnings yesterday.

The orchestra management had no official response of Rostropovich, who also walked the picket line with the musicians for nearly three hours on Monday. Credit for ending the possibility of police action was given to officials of the Park Police, the Kennedy Center, the symphony, the Inteorior Department, the National Endowment for the Arts and others.

Both sides have expressed the hopes that the strike will be settled in time for the orchestra to fulfil its engagements in Mexico next week. The deadline for that decision is tomorrow afternoon.

The musicians are striking over financial terms and the length of their contract.