President Reagan's summit with Soviet leader Mikhail S. Gorbachev hit unexpected snags Saturday on the eve of their talks as U.S. officials said prospects had faded for signing three secondary arms agreements and the Soviets harshly criticized U.S. aid to Afghan rebels.

The pre-summit uncertainty emerged as Reagan made final preparations for his trip to Moscow on Sunday and as the Presidium of the Supreme Soviet ratified the medium-range arms control treaty.U.S. officials said chances had evaporated for signing an agreement on advance notification of ballistic missile tests by the superpowers. Moreover, they said two lesser arms-control measures may not be ready for signing, as had been hoped.

Although none of the three agreements would have been considered major arms-control advances, they would have given the leaders grounds to boast of progress on arms control issues - particularly in view of the failure of the superpowers to conclude a treaty to curb strategic nuclear weapons.

Aside from the arms agenda, officials said they did not expect "announceable progress" on differences on human rights and regional disputes.

In a development that could further dampen the climate of the Reagan-Gorbachev meetings, the Soviets hinted Saturday that they might halt their withdrawal from Afghanistan if neighboring Pakistan failed to stop aiding Afghan rebels.

The Foreign Ministry statement, carried by the Tass news agency and the evening news show Vremya, took aim at the "lavish supplies of U.S. arms to Pakistan, intended specifically for the anti-government Afghan forces."

The United States has been supplying Afghan rebels with an estimated $500 million annually in arms and other military supplies channeled through Pakistan. It has pledged to continue to do so as long as the Soviet Union keeps up its military support of the Afghan government.

Nevertheless, Reagan was described as upbeat, rested and "very well prepared" for the talks, which begin Sunday afternoon at the Kremlin and continue until Thursday, when Reagan leaves.

Although there were new hangups in reaching agreement on the secondary arms agreements, Reagan on Saturday predicted that the two superpowers eventually will conclude a treaty to ban the most dangerous weapons in each other's arsenals - the long-range, or strategic, missiles.

"I don't think either of us have gone this far with the idea that it wasn't a good idea," the president said.

And Lt. Gen. Colin Powell, the president's national security adviser, predicted there would be "some additional movement - not a breakthrough" toward the Strategic Arms Limitation Treaty (START) pact. Powell said he still believes there's a possibility the accord will be signed before Reagan leaves office next January.

Powell suggested that progress was likely during the week on ways of keeping track of two elusive strategic weapons: land-based mobile missiles and cruise missiles fired from aircraft.

He also said the United States has some ideas worthy of discussion for resolving the residual ambiguity from the Washington summit over what type of Star Wars tests should be permissible. That is a major obstacle to a strategic arms pact.

With the absence of any significant agreements, the summit seems likely to be capped by another celebration over the treaty Reagan and Gorbachev signed last December to eliminate intermediate-range nuclear missiles, the first superpower arms treaty in 16 years.

The agreement was ratified 93-5 by the Senate Friday and approved unanimously Saturday by the Soviet collective presidency, permitting Reagan and Gorbachev to participate in a ceremonial exchange of ratification papers.

White House chief of staff Howard Baker left Washington for Helsinki on Saturday afternoon to carry the treaty documents to the presidential party. There will be a ceremonial exchange of the documents in Moscow on Wednesday.

As for the problems on the lesser arms agreements, Baker told reporters at Andrews Air Force Base before his flight that he was not afraid the president would come back empty handed.

"So much has already been achieved, that the importance of the meeting between President Reagan and General Secretary Gorbachev in Moscow is so important standing alone, that nothing will seriously detract from its importance," Baker said.

Reagan spent his final day before the summit in the privacy of his government guest house, holding two hours of talks with his senior advisors. The president also was shown on Soviet television, before an audience estimated at 200 million people, in an interview with Soviet journalists.

Reagan said he could not have envisioned a trip to Moscow under the Soviet leadership before Gorbachev.

"Very frankly, I have to say I think there is a difference between this general secretary and other leaders of your country . . . I don't think they had any dreams of perestroika" - Gorbachev's program to restructure the nation's troubled economy.

Reagan also expressed hopes for "more openness and the allowing of people to practice religion in the ways they choose" in the Soviet Union.

Before leaving Washington, U.S. officials had said three arms agreements probably would be ready for signing at the summit.

However, Assistant Secretary of State Rozanne Ridgway said Saturday that the Soviets had turned down a U.S. proposal for pre-notification of missile test launches.

She indicated the Soviets wanted a more sweeping "confidence-building" procedure that was unacceptable to the United States.

Powell said it was unlikely that two other agreements on monitoring underground nuclear explosions would be ready.

One of the expected agreements was a new protocol for on-site measuring of "peaceful" nuclear test explosions. Powell said the work had been completed, but negotiators were having trouble reconciling the Russian and English versions of the pact.

The other agreement was to have been a memorandum on ways to monitor experimental nuclear tests in Nevada and at Semipalatinsk in the Soviet Union. Powell said "a few technical issues have to be resolved."

Reagan set the tone for the summit talks during his weekly radio address, taped before he left Washington on Wednesday.

"We're ready to work with the Soviets, to praise and criticize and work for greater contact, and for change. Because that is the path to lasting peace, greater freedom and a safer world," he said.

In his television intereview, taped in the Oval Office for broadcast later, Reagan said he had read Gorbachev's book, "Perestroika," and that the goals it outlined "would reduce some of the differences between us further."

Reagan was challenged by one of the interviewers to document the source of his oft-stated claims that Vladimir Lenin had espoused a goal of Soviet expansion worldwide.

The president said "I can't recall all of the sources from which I gleaned this . . . (but that every leader except Gorbachev) reiterated their allegiance to that Marxian theory that the goal was a one-world communist state."

Putting in a plug for Vice President George Bush, Reagan said he was sure Soviet-American relations would continue along a path toward stabilization if Bush were elected. "I think that our people want this."

Reagan defended the human rights record of the United States but acknowledged that some people have prejudices against others. "But if they do anything to hurt that person because of that prejudice, the law takes care of them," he said.

Meanwhile, White House communications chief Tom Griscom said the United States, at the request of the Soviets, had provided the Kremlin with a list of dissidents and "refuseniks" invited to meet with Reagan at the U.S. embassy in Moscow on Monday.

He said he was aware that "a couple" of the invited guests had run into trouble with the Soviets about attending. However, Griscom said the embassy had been assured "that the people the president wants to meet with would be there."