Foreign Minister Agha Shahi says he will return to Washington shortly to complete a $2.5 billion U.S. military and economic aid package for Pakistan that he believes will move through Congress with little trouble.

Shahi said in an interview Friday that he expects "some anti-Pakistani lobbies" will try to "exploit Pakistan's urgent need for some items of defense equipment for their own purposes." He maintained, however, that "Congress would not discriminate against Pakistan."

Although the Reagan administration has not yet released the exact figures on the aid package being discussed with Pakistan, Shahi said a $2.5 billion total, spread over five years, would include about $400 million in military sales credits and $100 million in economic assistance annually.

Last week, the administration asked Congress for an initial $100 million in security-related economic assistance, despite legislative prohibitions against aiding countries, such as Pakistan, that are believed to be pursuing nuclear-enrichment technology.

Congress is considering an amendment to that prohibition, under which all U.S. aid to Pakistan was cut off two years ago. If it passes, the amendment would allow the president to waive the prohibition on national security grounds.

The administration request came shortly after Sen. Alan Cranston (D-Calif.) warned that the Indo-Pakistani arms race is speeding up and that both nations might explode nuclear devices in the next 18 months. For India, it would be the second such explosion. New Delhi set off its first nuclear device in a 1974 test and some U.S. officials fear that despite its denials Pakistan is seeking to gain similar nuclear capability.

Shahi returned here one week ago after extended talks with the administration in Washington. He said he also met with congressional leaders, including Sen. Charles Percy (R-Ill.) and Rep. Melvin Price (D-Ill.), respectively chairmen of the Senate and House foreign affairs committees.

He said he was "hopeful that Congress will accept the arguments put forward by an administration conscious of the fact that Pakistan, which he said was innocent of harboring any plans for developing nuclear weapons capability, was singled out for application" of the prohibition "while the real sinners were allowed to escape." The "real sinners," Shahi said, were countries such as India and Israel.

Shahi called the proposed military sales package a "modest one," but refused to say precisely what Pakistan wants to buy. Government officials in Washington have said there is still some disagreement on the issue, with Pakistan by all accounts specifically interested in F15 and F16 jets.

Shahi said, however, that the American government has turned down a request that the United States sell Pakistan weapons at the same cut rate that India was able to agree on with the Soviet Union last year.

"We were informed that the administration did not have the authority to supply military equipment at reduced prices," he said.

Shahi strongly denied that the package constitutes a major change in American-Pakistani relations.

"The relationship is an old and continuing one of friendship and cooperation which admittedly has seen its ups and downs," he said.

The only change is that in contrast to the Carter administration "the new administration was and is more keenly aware of the dangers inherent in the situation in the region and has shown greater interest in making a contribution in more realistic terms toward reaching Pakistan's security needs," he added.

He said Washington has understood Pakistan's policy of refusing to act as a conduit for arms for rebels fighting against Afghan and Soviet troops in Afghanistan. He has not been pressed on the point, he said.