Soviet Leader Leonid Brezhnev today stiffly reaffirmed the Kremlin's long-term military intervention in Afghanistan, while the Soviets elsewhere disclosed that the announced partial troop withdrawal includes one division of soldiers and 108 tanks.

"We had no choice but to send troops. And events have confirmed that this was the only correct decision," Brezhnev declared in a speech at the closed-door Communist Party Central Committee plenum made public later today by the official Tass press agency. "We will further help Afghanistan build the new life and preserve the gains of the April revolution . . ."

Radio Moscow, in a midday broadcast, said, "One Soviet division and 108 tanks are leaving Kabul," and quoted the Afghan Information Ministry as saying they are "leaving across the Salang Pass." Western sources here say this apparently means one regiment from an armored division and elements of a motorized rifle division, estimating that perhaps about 10,000 troops are headed back to Soviet territory.

But neither Brezhnev nor any other Soviet leader today disclosed whether the pullout means an actual reduction in the Soviet military force in Afghanistan. U.S. officials pointed to strong indications in recent days that the Soviets have been reinforcing their units, estimated earlier in the year to total about 85,000.

In other actions, the party plenum reaffirmed Brezhnev's leadership as president and party general secretary and that of his 76-year-old premier, Alexei Kosygin, by naming them as key speakers at the next full Communist Party Congress. Today the Congress' opening was set for Feb. 23. The plenum, consisting of about 300 voting Central commitee members and alternates, also declared that deteriorated world conditions, which they primarily blamed on Washington, "require all-around strengthening of the defense potential of our state in order to frustrate the plans of imperialism aimed at reaching military superiority and establishing a world diktat."

Significantly, Brezhnev in his brief address did not describe the Afghan withdrawal in any terms that could be interpreted as a gesture by the Soviets toward a possible political settlement of the crisis. Soviet troops entered Afghanistan Dec. 27 to install Babrak Karmal as head of the Kabul government.

Even though the troop withdrawal seemed to be timed for impact on the economic summit of seven key Western leaders that ended today in Venice and to help Moscow's boycott-damaged Olympics, which open July 19, Brezhnev made no reference to these things. He said the withdrawal is based solely on what he painted as Red Army success in achieving control despite American and Chinese subversion.

"Life in Afghanistan is now gradually coming back to normal," Brezhnev said. "Large gangs of counterrevolutionaries were routed. The interventionists have suffered a serious defeat. In these conditions we decided to withdraw from Afghanistan some units of our military contingent."

"We are doing this on agreement with the Afghan government and its head, comrade Babrak Karmal," Brezhev said of the withdrawal. "Of course we will further help Afghanistan build the new life and preserve the revolution's gains."

The Soviets are pressing other Moslem Third World countries and the Western nations to recognize Babrak as the legitimate Afghan leader, which would go a long way toward achieving the Kremlin's aim of defusing the issue.

Brezhnev restated Moscow's position that Washington and Peking, not Moslem tribesmen's hatred of the Afghan government, led to the use of Soviet troops. He said, "Imperialist policy to create a threat to our country from the south has failed," and accused Pakistan of allowing "bandit units" to cross its border to enter Afghanistan on raids.

The Soviet leadership ignored the call from the Venice summit yesterday for full Soviet withdrawal. But the Soviet media attacked it and said again that Iran and Pakistan should accept the May 14 Kabul initiative, to recognize Babrak and begin regional talks.

The plenum resolution backed detente, saying "real preconditions exist for preserving it as the dominant tendency in world politics," but blaming the United States and the North Atlantic Treaty Organization for trying to gain military superiority over the Soviets.

In looking to next year's party Congress, which will set the national growth plan through 1985 and must deal with major new economic problems, Brezhnev said the Central Comittee must find an answer to the "key problems" of energy and transportation squeezes and agricultural short-comings. pLabor productivity, work-place discipline and technical progress must improve, he said, to insure the "god things for the 1970s brought us" be made even better.

The last party Congress was in 1976, and the economy has fallen well short of achieving its expansion goals.