Muammar Qaddafi said tonight he believes President Reagan "has backtracked from war," but the Libyan leader vowed to wage his own economic fight to retaliate for the U.S. boycotting of Libya and freezing of its assets.

"The Libyan people are now waging economic warfare and should fight America with its own weapons," Qaddafi told a televised "popular congress" in Tripoli. "America has taken economic steps. We must take material steps, each one of us."

The precise measures Qaddafi has in mind were not elaborated. In recent interviews he has said that he would consider freezing U.S. assets in Libya, but he has made no clear statement on the issue. Rumors in Tripoli this evening suggested he might prohibit American-owned companies from lifting Libyan oil.

In anticipation of an Arab League foreign ministers' meeting planned for later this month, Qaddafi said, "We should ask the Arabs to support us in taking economic steps." But he cautioned, "We must be at the forefront of the battle to take positive action."

Dressed in a turban and a gold-edged cloak, Qaddafi spoke from the back of the audience in the auditorium where the congress was taking place, presumably symbolizing his role as one man among many.

Qaddafi seemed intent on preparing his population for austerity measures and chided the audience for concerns about the chronic shortages of food and consumer goods that were abundant in the past.

But he sought to assuage the fears of a U.S. invasion or air strike that have built up in the weeks since the Reagan administration announced its intention to retaliate for what it described as Libyan support for the terrorists who attacked the Rome and Vienna airports on Dec. 27.

"Reagan has backtracked from war," Qaddafi said. "The threat of war is still there, but if war comes I believe it will be in the sea hundreds of miles away from land."