A U.S. magistrate in Philadelphia held a Pakistani native in jail without bond yesterday on charges he sought to illegally procure U.S. material for Pakistan's nuclear weapons program. The charges triggered a major new issue in U.S.-Pakistan relations and a demand in Congress that all U.S. aid to that country immediately be cut off.

Arshad Z. Pervez, now a Canadian citizen, was charged with paying off a U.S. undercover operative as part of an extensive plot to export special maraging steels -- alloys of extremely high strength -- that U.S. Customs officials said would only be of use in Pakistan's clandestine nuclear weapons program.

Following the court filings and a State Department briefing, Rep. Stephen J. Solarz (D-N.Y.) demanded the Reagan administration take action under 1985 legislation, bearing his name, which mandates an aid cutoff if a non-nuclear country such as Pakistan attempts illegally to export nuclear weapons material from the United States.

U.S. military and economic aid to Pakistan totals $638 million this year.

Solarz, chairman of the House Foreign Affairs subcommittee on Asian and Pacific Affairs, gave major help to the administration in a hard-fought congressional debate earlier this year when he insisted that the large-scale U.S. aid program to Pakistan continue despite intelligence reports that Pakistan was continuing its drive to manufacture atomic weapons.

"Pakistan has abused our trust and violated our laws," Solarz said after learning of the arrest. "As much as I value America's relationship with Pakistan, I value American devotion to the law even more."

A senior State Department official said yesterday the United States will demand an explanation from Pakistan in an "intensive dialogue" on the case, which is to begin shortly. Calling it "a very serious matter" involving U.S. national interests, the official said there should be "no rush to judgment" on whether an aid cutoff is required.

Under the Solarz legislation, an aid cutoff would be triggered by a presidential determination that Pakistan was seeking to illegally acquire by U.S. export "material, equipment or technology" to make a nuclear weapon. The president could keep aid flowing, however, if he determines that cutting it off would be "seriously prejudicial" to U.S. objectives in combating nuclear weapons proliferation, or otherwise impair "the common defense and security."

The Carter administration had cut off U.S. aid to Pakistan because of the country's drive to acquire nuclear weapons material and technology illegally. But after the Soviet invasion of Afghanistan in December 1979, the U.S. submerged its antiproliferation concerns to Pakistan's importance as a front-line state bordering Afghanistan. Pakistan harbors about 3 million Afghan refugees, including the leaders of the Afghan resistance fighting the Soviet occupation.

In 1981, the Reagan administration suspended U.S. antiproliferation laws and put through a six-year, $3.2 billion Pakistan aid program, now in its final year. The administration has negotiated a new six-year, $4 billion aid program. In April, after heated debate, House and Senate committees adopted a Solarz formula approving a two-year waiver of antiproliferation laws so the new aid program can begin Oct. 1. The measure, however, is still pending in Congress.

The arrest comes as Pakistan, with U.S. encouragement, is seeking to acquire U.S. aircraft for radar surveillance of its Afghan border, which repeatedly has been violated by Soviet-supplied and Soviet-piloted aircraft supporting the Marxist regime in Kabul.

According to documents filed in U.S. District Court in Philadelphia, Pervez on Jan. 13 paid a $1,000 bribe to a U.S. Customs agent in Philadelphia posing as a Commerce Department licensing officer. David Warren, a Philadelphia customs official, said Pervez was not then arrested because U.S. authorities hoped to learn more by continuing the investigation.

Solarz said he had no indication the administration put off the arrest until after the April congressional debate on the Pakistan aid program. "My impression is that the administration has conscientiously pursued the case so far, with the Justice Department pursuing the violation of law and the State Department interposing no objections so far," Solarz said.

According to the court documents, Pervez sought to export 50,000 pounds of maraging 350 steel, a superhard alloy that could be used in a gas centrifuge plant such as the secret nuclear enrichment facility at Kahuta near Islamabad, Pakistan's capital. There has been speculation in Europe -- where Pakistani agents recently were caught trying to export nuclear materials -- that Pakistan seeks a major expansion of the Kahuta plant or the creation of a second atomic weapons facility.

The affidavit of U.S. Customs undercover agent John R. New also said Pervez asked him in June to help him obtain beryllium, a rare metal used in detonation of atomic weapons.

Pakistan consistently has denied seeking to make atomic weapons, claiming its nuclear program is entirely for civil uses. However, the program was begun after India, Pakistan's regional adversary, began preparations for a nuclear test explosion in 1974.

Court documents said Carpenter Technology Corp. of Reading, Pa., contacted U.S. Customs last November after receiving an inquiry from Pervez about purchasing maraging 350 steel, which had been the subject of special U.S. controls because of its potential use in nuclear weapons manufacture. Customs then inserted New, its undercover agent, into the negotiations with Pervez.

Statements by Pervez and documents seized late last week in Toronto under Canadian court order indicate Pervez was working for a retired Pakistani brigadier general, Inam ul Haq, who heads an export-import firm in Lahore. According to testimony in magistrate's court yesterday, letters from Inam appealed to Pervez to procure the materials in the national interest of Pakistan rather than simply for profit.