Negotiations between the National Football League and the striking NFL Players Association slowed to a virtual standstill today as representatives of both sides prepared to begin bargaining on the key economic issues. A planned night bargaining session was called off.

As the strike ran through its 28th day, there was no settlement in sight and it appeared all but certain that Sunday's NFL games would have to be called off.

That would be the fifth consecutive weekend of NFL games called off, and it is the last weekend of games that can be called off and still permit playing of a 13-game season. NFL Commissioner Pete Rozelle has said many owners claim there would have to be at least 13 games to preserve the credibility of the season; he also has said a 12-game season might be possible.

The two sides in the dispute met together for one hour this morning, then broke off for caucuses and separate meetings with mediator Sam Kagel.

At a late afternoon press briefing, Kagel said bargaining would resume at 7:30 tonight. But later that time was set back to 8:30, then 9:30, then finally to 9 a.m. Tuesday by Kagel. "Everyone," he said jokingly, "is watching the all-star game (put on by the NFLPA in Los Angeles)."

The delays were all requested by the union, angering management. "We've been ready to negotiate," a management council spokesman said. "We are ready to negotiate now. Time is not a luxury."

Kagel had said the talks would concern both economic and noneconomic issues, although agreement had been reached, at least in principle, on most of the latter.

Asked to confirm reports that both sides were pessimistic about the prospects of a settlement, Kagel said, "I can't account for anyone's state of mind."

When talks here recessed early this afternoon, both Ed Garvey, executive director of the NFLPA, and Jack Donlan, the NFL's chief labor negotiator, left separately from the grounds of the Hunt Valley Inn where the mediation is being held. Garvey went to Washington, reportedly to update player representatives on the negotiations. When he returned tonight, he was asked if his Washington meeting would help bring a quick end to the strike, Garvey replied, "I think it will."

To the question of whether the NFLPA was changing its position on the major issue of a wage scale, Garvey replied, "No."" Never?" someone asked. "I didn't say wouldn't," he said.

His meeting was not with Rozelle or any NFL owners, union sources said.

Donlan returned late in the afternoon.

Both management and union sources said the bargaining process, which began here Tuesday under Kagel's mediation auspices, was in great difficulty. But neither side was reported to be ready to break off the negotiations.

As it has been since February, the major stumbling block toward a settlement remains the players' demand that a trust fund be set up to pay them on a seniority-based scale with performance incentive bonuses.

Management says it opposes that concept and says instead it wants to keep the current system of individual negotiations.

Earlier today, Garvey termed the union's so-called all-star game at RFK Stadium Sunday a great success, despite a gate of only 8,760. "I don't think it affects what is going on with the mediator, but we're just extremely happy that the game was as successful as it was," Garvey said.