Talks between the government and the Professional Air Traffic Controllers Organization recessed abruptly yesterday with the two sides still more than $600 million apart, leaving little hope of avoiding a nationwide strike scheduled for 7 a.m. Monday.

A strike, illegal for the federal workers, could affect 50 to 75 percent of the nation's 14,000 scheduled daily flights. Coming about two days before Congress is to begin its August recess, it also could strand some lawmakers in Washington.

But airline officials, who drew up contingency plans a month ago when the controllers were negotiating a tentative pact, say they are better prepared this time and do not expect wholesale service interruptions here.

Passengers using National Airport, they say, should be able to travel with a bit of schedule-juggling under a "flow control" or slowdown plan that might be implemented by the Federal Aviation Administration.

Eastern Airlines spokesman June Farrell said she expects the bulk of the airline's 90 daily flights in and out of National to continue. A U.S. Air spokesman said passengers with confirmed reservations on that airline should be able to continue their travel plans, although they may have to switch to other flights.

The two sides met for 30 minutes yesterday under the auspices of the Federal Mediation and Conciliation Service before recessing talks until 2 p.m. today. Kenneth Moffett, acting director of the mediation service, said he doubted that today's session would bring a timely settlement.

"When you're $681 million as opposed to $40 million apart, that's quite a chasm," Moffett said. "We hope we can get something cooking tomorrow. But it doesn't look good right now."

Meanwhile, warnings against a strike came from the administration and Capitol Hill. Transportation Secretary Drew Lewis said the administration "would bring the full force of the Justice Department down" on any controllers who walk off their jobs. And Sen. Jake Garn (R-Utah), who last week sent a warning letter "to all air traffic controllers" that was signed by 54 other senators, said in an interview yesterday that he would filibuster against congressional approval of any agreement "gained by an illegal work stoppage." Congress has the final say on any new pact with the controllers.

The current round of bargaining was prompted by the PATCO membership's 95.3 percent vote last week against a tentative agreement between the union and the Federal Aviation Administration. The tentative pact, signed June 22, would have given the nearly 15,000-member union a $40 million increase in wages and benefits.

But PATCO members, acting on the recommendations of their executive board and their president, Robert E. Poli, rejected the contract as failing to meet their primary demands: a 32-hour workweek and an improved retirement plan.

Lewis on Friday asked Poli to delay the new round of bargaining at least a week. Poli refused.

Poli gave Lewis a revised list of demands Friday that the union estimates would cost about $500 million. Lewis said the cost of what he said were 99 union proposals would be about $681 million annually.

Lewis repeated yesterday that he will not budge from the $40 million offer the Reagan administration made in June.

"When you're that far apart, 17 times apart, there's no sense in budging from anything," Lewis said.

Poli said, "They've refused to give us a counterproposal . . . Without a counterproposal, the chances are very good" for a strike.

The PATCO president said Friday that if there was no new tentative agreement, he would order a strike vote to begin at midnight tonight, and a strike would begin at 7 a.m. Monday if 80 percent voted in favor of the action.

However, a usually reliable FAA source told The Washington Post yesterday that Poli will use the contract rejection vote as his strike authorization, avoiding the trouble of taking a full vote tonight.

"They are not going to take a new vote," the source said. The allegation was termed "highly believable" by a Reagan administration official who requested anonymity, but a PATCO press aide, Marcia Feldman, dismissed the charge as "totally untrue" and "just another one of many rumors."

In a related development yesterday, the 500-member National Black Coalition of Federal Aviation Employes, representing controllers and other FAA workers, sent Poli a telegram urging him to resolve outstanding differences through arbitration. Joseph Cassells, the group's president, said his organization's policy "is not to break the law" through participation in an illegal walkout by federal employes.