Soviet President Leonid Bezhnev declared today that relations with the United States can be improved sharply if the future Reagan administration adopts "a well-considered, realistic policy" toward Moscow, including new efforts at limiting strategic arms.

The Kremlin leader made the assertions during a 2 1/2-hour meeting with Sen. Charles Percy (R-Ill.), the incoming Senate Foreign Relations Committee chairman.It was Brezhnev's first face-to-face session with a senior U.S. politician since last December's invasion of Afghanistan consigned bilateral detente to the deep freeze.

According to the official Tass news agency, Brezhnev told Percy there "should be no stagnation" by the superpowers in trying for new strategic arms talks.

"Success in this matter lies at the heart of all international security," Tass quoted Brezhnev as saying. But Percy, who laid out the general foreign policy views of President-elect Ronald Reagan after detailed consultations in Washington, indicated at a press briefing afterward that the discussion did not include exploration of Reagan's promise to scrap the unratified second strategic arms limitation treaty and seek to reopen the arms talks.

That issue may arise Thursday, when Percy is to see Defense Minister Dmitri Ustinov, and Friday, when he is to talk with Foreign Minister Andrei Gromyko. The two capitals have had little high-level contact since Moscow's intervention in Afganistan.

Neither Tass nor Percy said whether Brzhnew specifically addressed his remarks to Reagan, but no veteran observer here doubts that he was the intended listener. In the view of one seasoned analyst, "Brezhnev offered the usual armored olive branch."

Publicly, the Soviets say SALT II must be ratified as signed by Brezhnev and President Carter in 1979. Privately, it is known, Soviet officials are interested in hearing what Reagan proposes to do to the treaty.

Percy said the meeting was marked by Brezhnev's "realistic, pragmatic" presentation of the Soviet position on issues dividing the superpowers. He said he is opimistic about future relations as a result of the meeting.

[The senator told United Press International, "I have heard . . . they will invite me to address the Soviet people on open television sometime, I think tomorrow or the next day." U.S. Ambassador Thomas Watson was refused television time July 4 because his talk touched on Afghanistan.]

Tass quoted Brezhnev as saying the two countries "have to find a common language" to deal with other "pressing international problems. There is no lack of readiness on the part of the Soviet Union to have dealings with the U.S. in a constructive manner, with mutual account taken of each other's ligitimate interests." The Soviets do not accept the notion that the United States has legitimate interests in the Afghan situation, although Moscow accuses Washington of trying to subvert the Kabul Communists.

"We did not and do not want now any worsening of relations with the U.S.," Brezhnev said. "We do not want all the positive things that were achieved by joint efforts earlier to be thrown back." Moscow wants good long-term relations with Washington, he said, and will not allow it to gain military superiority.

"The Soviet Union is not the kind of country to be talked to on such a basis," he added. "We could well cooperate as equal partners if the American side also displays readiness for this."

Percy showed enthusiasm for the meeting, expressing satisfaction that he could serve as a go-between.