Union leaders said yesterday they hope several thousand workers, representing more than 200 area labor organizations, will heed their call to support striking Redskins football players and take part in picketing and a rally at Robert F. Kennedy Stadium tomorrow.

Local leaders of the AFL-CIO, with which the striking NFL Players Association is affiliated, said they will establish a picket line outside the main stadium entrance before the scheduled game between substitute players representing the Redskins and the St. Louis Cardinals.

Although union executives said they still had no clear idea of how many union members would participate, they hoped for a massive display of protest against NFL owners, with local workers being joined by labor organizers from as far away as Cumberland, Md.

"We think we will have more people out here than in there," said Joslyn N. Williams, president of the Metropolitan Washington Council, AFL-CIO, pointing to the stadium during a news conference outside RFK yesterday.

Union officials said they would make no attempt to prevent the game, scheduled to start at 1 p.m., from being played.

The D.C. Armory Board, which operates the stadium for the city, has designated that picketing is to be conducted in an area about 50 yards from the stadium. A rally billed by organizers as a family event, with music and autograph signing by striking Redskins, is planned during the game on a nearby grassy strip.

Stadium officials said they foresee no major disruptions of services at RFK tomorrow. But concessionaires were scrambling to find replacements for possibly 300 or more unionized beverage and food vendors who are expected to honor the players' picket line. They called on city high schools, looking for stand-ins to hawk beer and hot dogs.

Truck drivers who deliver beer to the stadium have pledged their support for striking players along with other members of the Teamsters Union. But this Sunday's allotment of brew is already on hand, trucked in when no picket line was in place, according to Teamsters union officials.

Nearly all of the approximately 1,000 ticket takers, ushers, security guards, grounds crew and police are expected to report to work as usual, said James Dalrymple, president of the D.C. Armory Board.

But Dalrymple said that for the first time in 21 years, the stadium will not be sold out for a Redskins game. Thousands of regular ticket holders have asked for refunds, and Dalrymple predicted a crowd of 35,000 to 40,000, compared to the usual capacity audience of more than 55,000.

Metro bus and rail service to the stadium also is expected to run on schedule. Transit workers, who voiced their support of the players, said they will not interfere with the area's transportation network.

With NFL owners making good on their threat to continue the regular season with substitute players, union leaders here and across the country have rallied around the striking players, saying the dispute symbolizes trends in labor-management relations in and out of sports.

In Metro bus garages, in grocery stores, government offices and elsewhere throughout the region, notices were posted this week to tens of thousands of union employes of all kinds, seeking a show of strength on the players' behalf.

"I expect that the outcome of this will have significant repercussions for labor-management relations in the future," said William Burrus, executive vice president of the American Postal Workers Union.

D.C. Mayor Marion Barry issued a statement Thursday saying he would not cross the players' picket line to attend games at RFK as long as the strike continues. Players have made the rounds at the District Building seeking support from D.C. Council members, with varying results.

Member James Nathanson (D-Ward 3) said he would not attend games and that he had asked people to whom he had already given his official allotment of game tickets also to honor the picket line.

"I'm not saying anybody is right or wrong on the issues. But I am still a member of a union and just as a matter or principle, I try to respect a picket line," said Nathanson, a former D.C. schoolteacher.

William H. Simons, president of the Washington Teachers' Union, said he expects to be joined on the picket line Sunday by at least 200 city teachers, who he said are not deterred by the large disparities in pay between teachers and football players.

"They're not bothered by the salaries," Simons said. "They recognize that the issues are valid issues."

The Maryland-D.C. chapter of the AFL-CIO giant this week sent notices of the picketing plans to 418 locals representing about 400,000 members. An annual meeting here of the chapter's executive board is being postponed tomorrow so that board members can join the picket line at RFK.