Libya's Col. Muammar Qaddafi has moved away from earlier efforts to subvert Tunisia's government and is now cooperating in lowering tensions in the northwestern region of Africa known as the Maghreb, Tunisia's Prime Minister Mohammed Mzali said yesterday.

"He has told us he wants better relations, and we have taken him at his word," Mzali said in an interview at Blair House, where he is staying during an official visit that includes meetings with President Reagan and other senior officials. "We hope that this continues, even if we know that in politics you can never be absolutely sure of anything."

In seeking to win congressional approval this month for a major increase in low-interest loans under the Foreign Military Sales financing program for fighter aircraft and tanks for Morocco and Tunisia, the Reagan administration has stressed the security threats to the region posed by Libya. Peter D. Constable, a deputy assistant secretary of state, told the House subcommittee on Africa last week that Tunisia is "under direct threat from Libya and inadequately equipped militarily."

Mzali did not directly dispute this assessment, but he portrayed the $140 million reequipment program for Tunisia that the Reagan administration seeks to finance in far more prosaic terms. "We have devoted in the past years all of our spending to development, to education, to health, to agriculture, and neglected the military. It is like moving into a new house and then waiting 10 years to get any furniture, or even a door. We need a door. We need to be able to protect our coasts, where now we can't even prevent foreigners from fishing our waters.," he said.

Opponents of the sharp increases in military sales to Morocco and Tunisia assert that the United States is shifting the emphasis of its aid to the region away from economic assistance and food aid under the Public Law 480, or Food for Peace, program toward selling military hardware to governments that face internal problems due to badly sagging economies, rather than direct external threats.

Mzali, 56, has been a leading figure in President Habib Bourguiba's Destour Party and in successive governments for more than two decades and is considered in Tunis as a leading candidate to inherit power from the 78-year-old president.

One of the focal points of discontent in Tunisia in recent years has been spiraling unemployment rates both for university graduates who find no work and for workers who have been shut out of jobs in Western Europe because of the recession there. Mzali acknowledged that at best the new five-year economic development plan, which calls for the creation of 60,000 new jobs a year, would fall 10,000 short.

There are an estimated 60,000 Tunisians working in Libya, and Qaddafi appears to be attempting to woo Bourguiba's government with economic concessions after evident Libyan involvement in a paramilitary uprising in Gafsa in January 1980 soured relations between Tripoli and Tunis. The Libyan leader has visited Tunis twice this year.

The Reagan administration is seeking a $100 million in Foreign Military Sales credits for Morocco, a two- to three-fold increase over average amounts provided in the past five years. The Tunisian Foreign Military Sales request, which Mzali indicated that Tunisia was still trying to get on softer terms, is a $55 million increase over last year. Both programs envision the sale of M60 tanks and F5 fighters to the two Arabic-language nations.