Soviet President Leonid Brezhnev yesterday watched a mock battle by artillery, tanks and planes in a show of force close to the Chinese border that is a clear gesture to Peking of Soviet ability and determination to guard the disputed area.

Brezhnev was in Khabarovsk in the Soviet far east, about 25 miles from the Chinese border. The area is one of the most sensitive along the lengthy scene of fierce Soviet-Chinese clashes on the Ussuri River in 1969.

Brezhnev has been traveling by private train along the Trans-Siberian Railroad for the past eight days, accompanied by Soviet Defense Minister Dmitri Ustinov.

According to Tass, the official Soviet news agency, Brezhnev witnessed a tactical exercise involving motorized infantry, tanks, artillery, warplanes and antiaircraft batteries. Tass emphasized both the moderness of the equipment and the firepower.

Brezhnev "noted good cooperation between" the various units, the agency said. The Tass account seemed to imply that these were not simply lightly armed defensive border units, but well-mechanized, reinforced armored infantry capable of offensive operations.

Brezhnev is chairman of the Soviet Defense Council as well as president and leader of the Communist Party. His tour comes at a time when the Soviets have gone out of their way to offer descriptions of how Peking has rebuffed repeated attempts by the Kremlin to negotiate their border dispute and other bitter differences that have arisen between the two Communist powers.

On Saturday, for example, a Pravda editorial described in unusual detail what it said were Chinese demands that the Soviets withdraw their troops from thousands of square miles in the Khabarovsk area, in effect ceding territory to the Chinese. The demands also focused on removal of Soviet troops from Mongolia. The border dispute has simmered for years and a joint Soviet-Chinese commission set up in 1969 has failed to reach an agreement. Western military sources have estimated the Soviets have more than 500,000 troops along the border.