The Soviet Union stepped up its propaganda campaign against the Chinese invasion of Vietnam today amid an unconfirmed report here that the Red Army has been placed on a national alert, with leaves canceled and soldiers recalled to their units.

The intensified media campaign included battlefield accounts in the newspapers and on television, angry letters, and wide publication of statements of outrage over the invasion by many world capitals.

The propaganda blitz seemed to reflect decisions taken before reports reached here that Chinese Vice Premier Teng Hsiao-ping declared in Peking that the Chinese invasion had ceased as of today. A television commentary tonight, in what seemed to be a sign of restraint, suggested that the invasion may be brief.

Commentator Vladimir Dunayev gave details of the attack not previously made public here and quoted Peking as saying the Chinese do not plan to hold on to Vietnamese territory. Dunayev predicted that a combination of Vietnamese resistance, worldwide protests and the Kremlin warning of yesterday will force Peking's withdrawal.

In any case, Western observers suggested that the Soviets now are sitting tight and seem unlikely to undertake any direct military intervention, especially if the Chinese attack indeed has halted and the invaders pull back behind their borders.

It was impossible to confirm the reported army alert and leave cancellation. The report circulated from Victor Louis, a Soviet journalist with wide official connections. He filed such a story with the newspaper he represents, the London Evening News.

The Louis article asserted that the army had been called to a state of high readiness. In conversations with Western journalists, however, he said he himself had no official confirmation of the story, but that his university student son had been told of the leave cancellation by a friend who had picked up "village gossip."

He said he filed the article because his paper was pressing him for a story.

Soviet military authorities contacted tonight refused to respond to reporters' queries, telling one Westerner, "We do not give out such information."

Reliable Western sources here said they had no word of any change in the readiness status of the army and had not seen or heard of unusual troop movements in the capital or elsewhere in the country.

Nevertheless, an actual army alert -- or even a spurious report that takes on a life of its own -- could do much to achieve the Kremlin's apparent aim to look tough, talk tough, but avoid acting tough and getting into a military action with the Chinese.

Qualified Western observers pointed out that the Soviets have already achieved an immense propaganda victory over their adversaries in Peking. They presumably would not want to harm their own international image by hostile military acts at a time of significant signs that the Chinese have stopped the invasion in the face of intensifying Vietnamese resistance.

In addition, none of the media effort changed the formulation of yesterday's Kremlin warning that China must stop its attack "before it is too late." The government has not specified what it might do to help its embattled ally if the Chinese invasion continues.

Soviet Foreign Minister Andrei Gromyko reiterated that warning today, declaring: "It should not be forgotten in Peking that Vietnam is not alone. It has many friends and allies in the whole world. We firmly declare that the present Chinese leadership should stop the aggression before it is too late. Our position was clearly outlined in the government statement published yesterday."

Gromyko's remarks, in a Kremlin meeting with Czechoslovak Foreign Minister Bohuslav Chnoupek, were reported by the official Tass press agency.

The Soviets have loosed a propaganda barrage over the invasion and today a commentator in the government newspaper Izvestia extended this effort to include the United States, deriding its recent hosting of Vice Premier Teng.

"The Americans now are making a point of emphasizing that they tried in every way to persuade their Chinese friends not to resort to extensive military actions on the border with Vietnam," wrote political commentator Alexander Bovin.

"Maybe that was the case. For the Americans did understand that a Chinese attack on Vietnam, especially immediately after Teng's visit, would place Washington and all the other Western 'appeasers' of China, in a very ambiguous and politically vulnerable position."

The Carter administration has called on the Chinese to withdraw and urged the Soviets and Vietnamese to exercise restraint in dealing with the invasion. The Kremlin, whose relations with the Chinese are severely strained, was deeply angered by Teng's anti-Soviet statements while he was in the United States.

Bovin accused Peking of seeking to wreck European peace, and asserted: "Peking did everything possible to undermine the development of Soviet-American relations and torpedo agreements on limitation of strategic arms."

Western analysts here have speculated that Soviet leader Leonid Brezhnev's desire to achieve agreement on a new SALT treaty with Washington has acted as a restraining influence on Soviet actions in the current crisis.

Tension has cracked along the long Soviet-Chinese border for years, where 44 Soviet divisions heavily outgun Peking's troops. There were serious armed clashes in 1969 over islands in the Issuri River, with both sides reporting inflicting heavy casualties on the other.

Last May, shooting erupted again there when Soviet border units apparently blundered into Chinese turf. The Kremlin apologized, but repeated attempts to negotiate the longstanding border dispute have failed.