For the first time since negotiators for the National Football League and the striking NFL Players Association entered mediation a week ago, talks today were exclusively on economic matters, the major obstacle to settling the 29-day-old strike.

But prospects for a settlement of the dispute deteriorated quickly amid disclosures that the NFL management negotiators have withdrawn their promise to guarantee a total salary package of $1.6 billion to the players over the next five years.

"The guarantee was based on the number of games played. That number has been reduced and our revenues have been reduced," said a spokesman for the NFL Management Council, the league's labor negotiating arm.

Ed Garvey, executive director of the NFLPA, said management's withdrawal of the guarantee proved his contentions all along that the promise was never valid.

Late tonight union sources said assistant coaches for the Washington Redskins and the Cincinnati Bengals had been calling players to inform them that training facilities would be open within two days and that players would be asked to return to work despite the continuation of the strike. The Management Council declined to confirm or deny that report, saying it had no knowledge of any such action.

The two sides met throughout the day but the night bargaining sessions were called off by mediator Sam Kagel. Management sources said consideration was being given to calling a recess within the next day or two to permit the sides to reevaluate their positions.

However, the union indicated it does not want to recess the talks. "We want to stay here and continue bargaining," a union source said. "Today is the first day we have gotten around to the issues that have caused the strike. There is some real give-and-take bargaining going on in there."

But management sources were considerably more pessimistic, saying that, at the moment, chances for a settlement appear to be slim.

In a formal prepared statement released last night the union said, "Management has been pushing hard for a recess and a so-called cooling off period. In 1974 we had a cooling off period and it was a disaster. It took three years to get an agreement." The union said it is unalterably opposed to any suggestions that its members return to work without a contract. It said its player representatives will meet next week.

Management sources said they had never suggested that the union return to work without a contract.

Meanwhile, the NFL called off Sunday's games for the fifth straight weekend.

At his afternoon press briefing, mediator Kagel said today's talks began at 9 a.m. and continued throughout the day.

"What we're doing now is negotiating and mediating all of the economic issues in great depth," Kagel said. "This is a serious discussion of the respective positions of the parties on each of the issues contained in the total economic package."

Sources close to the talks said Kagel has suggested that the bargaining process be moved to San Francisco, his home city, if the sides decide to call a recess within the next few days.

Kagel also played down reports that the talks have become deadlocked, saying, "As long as there is discussion, there is movement."

But there appeared to be little, if any, movement over the central isssue dividing the two sides: the union's demand that a trust fund be set up to pay the players according to a seniority-based scale with performance-incentive bonuses, a concept that management says it opposes.

Today marked the completion of a full week of efforts to resolve the strike under the mediation of Kagel, a 73-year-old lawyer and a veteran mediator and arbitrator of several West Coast labor disputes.

But after a period of relative progress in the first two days of talks, the pace slowed abruptly toward the end of last week and, by Monday had reached a virtual standstill. Although all the significant noneconomic issues have been resolved, there has been hardly a dent in the economic package.