Labor and government leaders in the deadlocked air traffic controller's dispute emerged from talks last night cautiously optimistic that a nationwide strike threatened for Monday morning could be averted.

Talks are to resume at 10:30 this morning.

"The fact that we're meeting together is progress . . . there is give-and-take," Transportation Secretary Drew Lewis said after a 2 1/2-hour session with Robert E. Poli, president of the Professional Air Traffic Controllers Organization. "Obviously when you're talking, you're talking because you want to avert a strike," Lewis said.

Poli, who earlier had accused the government of offering a "take-it-or-leave-it" package, said last night that "as long as the parties are still talking, there is some optimism involved."

Federal mediator Kenneth Moffet also said he was "more optimistic" that differences between the union and the Federal Aviation Administration could be settled without a strike. "I hope that we can reach agreement . . . this weekend," he said.

Nevertheless, the nation's commercial airlines continued emergency preparations in the event of a strike. Plans call for a drastic reduction in the number of scheduled flights, especially those of less than 500 miles.

Last night's meeting came at the behest of Chairman James J. Howard (D-N.J.) of the House Public Works and Transportation Committee toward the end of a full day of congressional testimony. During the day Poli was warned sternly by House members, including some who expressed sympathy for PATCO, that they would push for tough penalties against controllers -- federal employes -- who walk off their jobs.

Pressure to resume the talks also came from the Senate, where 36 senators, including 13 of the 17 members of the Commerce Committee that eventually will have to approve a controllers' contract, warned that they would take no action on a pact if there is a national walkout Monday.

"We want to make clear our position that illegal action will do nothing to further your goals of increased pay and changes in working conditions," the senators told Poli in a letter.

Also yesterday, the Reagan administration toughened its already hard stance against a PATCO strike. After briefing President Reagan on the situation, Transportation Secretary Lewis warned: "We will not tolerate an illegal strike, nor will we negotiate with the union while a strike is taking place." He said the administration first would seek civil action against PATCO, then criminal penalties if necessary.

Howard suggested tha tthe strike could be postponed if the two sides, with some moral support from the subcommittee, returned to the bargaining table. Poli told the subcommittee that walkout "possibly could be averted," even without a tentative agreement Sunday night, if there were "meaningful negotiations" and "we are close to an agreement."

In the event of a strike, the airlines will follow an FAA contingency plan based on a philosophy that travelers going shorter distances have options that travelers going longer distances don't have.

In the worst-case scenario painted by the RAA, most flights of under 500 miles would be canceled, while international and transcontinental flights would be relatively unaffected. However, the FAA has reserved the right to give U.S. and foreign airlines flying international routes four hours' notice not to take off, according to FAA Administrator, J. Lynn Helms.

Determination of which flights would operate depends on the number of controllers who actually strike government officials think some union members may not join a walkout -- but also to a great extent on the number of people who walk out of given centers.Helms said safety will be given the highest priority. "Safety will be maintained even if it means keeping more aircraft on the ground," he said.

The nation's airlines, which operate 14,000 scheduled flights daily, are warning travelers of the possibility of delays and inconvenience.

In an effort to help the airlines deal with the possibility of severe curtailment of scheduled flights, the Civil Aeronautics Board said that in the event of a strike it will suspend federal regulations requiring airlines to provide amenities to stranded passengers, guarantee no-smoking seats on every airplane and pay passengers with confirmed reservations who are bumped off overbooked flights.