MOSCOW, DEC. 13 -- The Soviet party newspaper Pravda today gave a glowing assessment of last week's summit conference in Washington, describing the signing of an intermediate-range nuclear forces (INF) treaty as "a watershed dividing the era of a growing nuclear threat from the era of the demilitarization of man's existence."

"Yes, reason triumphed," Pravda said in its regular Sunday international review column. "And while it is not yet the biggest victory, it has major significance both in political and in psychological terms."

The Soviet press, which yesterday fell silent on the summit's results, today said the meeting had produced a strong basis for future agreement on a proposed 50 percent cut in strategic nuclear weapons.

"The concluded agreements, which through collective efforts produced experience in this difficult business, give hope that ways will be found to solve even more difficult questions of equal and common security," Pravda said.

The party newspaper said the crucial issue in Washington was the agreement in principle to cut strategic arsenals by half, while both sides keep a "strict observance" of the 1972 Anti-Ballistic Missile Treaty.

The terms for observing the treaty, whether under the broad or narrow interpretation of the ban on the development of defense systems, are still vague, leaving in question the link between the U.S. Strategic Defense Initiative and the proposed strategic arms cuts, western diplomats here said over the weekend.

Pravda today carried on its front page a short item by the official news agency Tass, repeating without comment President Reagan's postsummit remark that the SDI program is not an obstacle to negotiations on cuts in strategic weapons.

While stressing the "collective" efforts behind the arms agreement, Pravda noted that the INF treaty eliminating medium- and shorter-range weapons was "a celebration" of "new political thinking," a term applied to Soviet leader Mikhail Gorbachev's foreign policy initiatives.

The same thought was echoed in Warsaw, where Moscow's new party chief, Lev Zaikov, a member of the Politburo, went for a one-day gathering of party leaders of the Warsaw Pact's capitals.

Zaikov called the INF agreement a "triumph of the new political thinking which our party laid as the foundation of its struggle to establish a lasting nuclear-free and non-violent world."

In a report from Washington today, Pravda said it was "characteristic" of the Washington summit that no serious observer or specialist was commenting on one side's or the other's concessions.

"On the contrary, they all noted the mutually acceptable compromises which alone were able to make the most important goal realizable," the report said.

In a dispatch from Washington, Pravda stressed the positive reaction to the summit, noting that the rise in popular support for the INF treaty will register in the U.S. Senate, which now will consider it for approval.

In signing the treaty, the two sides set in motion a complex series of steps that will lead to the destruction of 2,611 nuclear weapons.

All Soviet newspapers this week have reprinted the text of the treaty and, in commentaries, listed the numbers of warheads to be destroyed under the INF pacts, showing the greater number to be destroyed by the Soviets.

The disproportionate numbers have apparently raised some questions in the public mind since a television commentator, answering readers' mail, tonight quoted a letter that asked why the Soviet Union was destroying more missiles than the Americans.

The commentator reassured viewers that the step will not do any harm to the Soviet Union's security and then suggested that the greater number of warheads destroyed on the Soviet side could serve as an example for the Americans.