MOSCOW, FEB. 21 -- The Soviet Union's most renowned dissident, Nobel Peace Prize winner Andrei Sakharov, told Secretary of State George P. Shultz here today that serious reforms are taking place in Soviet society but he urged the United States to continue pressing Moscow to free all political prisoners and withdraw its troops from Afghanistan.

Shultz took time off from three long and apparently productive meetings with Soviet Foreign Minister Eduard Shevardnadze to pay a 45-minute call at the flat of the 66-year-old physicist and outspoken advocate of human rights. Sakharov was permitted to return to Moscow about a year ago after seven years of exile in the provincial city of Gorky.

Shultz, accompanied by his wife, Helena, and several senior State Department officials in an unusual gesture of U.S. respect, praised Sakharov as "a model and a symbol to the world."

Sakharov, whose work on Soviet weapons decades ago was instrumental in the development of the Soviet hydrogen bomb, told Shultz that "both sides should make important concessions toward each other" in negotiations to reduce their atomic weapons arsenals.

"I called on the American side to be restrained in its attitude toward SDI {President Reagan's Strategic Defense Initiative aimed at creating missile defenses in space} and to observe strictly the Antiballistic Missile Treaty," Sakharov told reporters following the meeting.

Sakharov added that he called for "the release of all prisoners of conscience from Soviet jails and the complete end to the war in Afghanistan." Although some prisoners were released in the past year under the policies of Soviet leader Mikhail Gorbachev, the human rights monitoring group Helsinki Watch estimates that about 400 political prisoners remain.

U.S. officials, meanwhile, reported some progress in virtually all areas of the wide-ranging discussions with the Soviet negotiators today, and the two sides said a joint statement is planned for Tuesday, announcing strides that have been made.

No details were made public pending final talks between Shultz and Shevardnadze on Tuesday and a Kremlin meeting of Shultz and Gorbachev.

Shultz and Shevardnadze today signed a reciprocal fishing agreement that gives U.S. fishermen access to the 200-mile economic zone off the Soviet Pacific coast. After the signing, Shultz called the accord "one more example of progress" in U.S.-Soviet relations.

State Department spokesman Charles Redman quoted Shultz and Shevardnadze as agreeing that the atmosphere of their discussions was "excellent and businesslike."

Shevardnadze was particularly optimistic in his remarks about the drive for a sweeping strategic arms treaty that would cut U.S. and Soviet arsenals by 50 percent. He told reporters as the meetings began that he sees "a good chance" such a pact can be completed in time for Reagan to sign during his planned summit meeting here in late May or early June, and U.S. officials said he repeated this assessment in his private discussions with Shultz.

A U.S. participant in the talks said Shultz discussed Washington's approach to verification of the proposed nuclear weapons cuts, and he urged that such questions be accorded urgent priority in the Geneva arms negotiations.

The Persian Gulf, Angola and the promised Soviet withdrawal from Afghanistan were discussed in depth by senior aides, and Shultz and Shevardnadze were reported to be addressing these issues and the Middle East in a meeting tonight.

On human rights issues, Shultz was quoted by aides as expressing satisfaction about "more systematic and satisfactory" procedures and methods for addressing these problems in the Soviet Union.

Shevardnadze reportedly expressed concern about the execution of minors and about homelessness and other social and economic ills in the United States.

Following his second meeting with Shevardnadze, Shultz met a group of Soviets with a wide range of human rights problems and assured them of the U.S. commitment to help them improve their conditions.

Shultz spent 20 minutes greeting Soviets who had been denied permission to join their American spouses, Jews whose requests to emigrate to the West had been denied and activists fighting for the release of political prisoners.

"I want to be sure you realize how strongly President Reagan, the United States and the American people feel that rights are the essence of our society," Shultz told the crowd of 80, gathered in an apartment on the U.S. Embassy compound. "No matter how discouraging it sometimes seems, we will never give up."

Meeting with Shultz, Sakharov discussed ways to improve the Soviet human rights record, but he also defended Gorbachev's drive for glasnost, or openness, and perestroika, or the restructuring of Soviet society.

"Perestrioka and glasnost have been developing in a serious way," Sakharov told Shultz, according to a U.S. official present during the meeting. "It's not cosmetic. It's aimed at certain needs in Soviet society."

Sakharov also appealed for further support for the Kremlin leader, whose reform drive is said to face opposition. "I would like to see a deepening and expansion of Gorbachev's position," he told reporters.