Leaders of Warsaw Pact nations gathered in the Grand Kremlin Palace today to heap praise on Soviet President Leonid Brezhnev on his 75th birthday, but what was originally planned as a festive celebration was turned somber by uncertain events in Poland and the absence of Polish leader Wojciech Jaruzelski.

Soviet anxieties about the Polish events were reflected in an intensified media campaign against the United States today, including a bitter attack on President Reagan.

The official news agency Tass assailed Reagan's suspension of economic aid to Poland during martial law as a "coldblooded calculation to take advantage of Poland's economic problems." It said the Reagan administration "intends to cause hunger in Poland and thus exert pressure on the Polish leadership."

A strongly worded article in the armed forces newspaper Krasnaya Zvezda (Red Star) insisted today that the Polish situation had to be resolved, and that there can be no going back to the situation that followed the establishment of the independent trade union movement Solidarity in August 1980. The article said the original Polish mistake was to "recognize Solidarity as a competitor to the Polish Communist Party."

Soviet dispatches from Warsaw continued to emphasize that the country was generally calm, although reports of disturbances have become more frequent in the past two days.

Well-informed sources here said that Moscow had sharply increased its food deliveries to Poland prior to the military takeover. They said that 30,000 tons of Soviet meat scheduled to be delivered between Nov. 1 and Dec. 31 had been shipped to Poland before Dec. 12.

The Soviets also are reported to be ahead of schedule in their deliveries of other foodstuffs, cotton and fertilizers.

Diplomatic sources said the top-level talks here among Warsaw Pact members Bulgaria, Hungary, East Germany, Czechoslovakia, Romania and Mongolia are expected to focus on economic assistance to Poland, presumably involving joint efforts to flood the country with food and consumer goods to shore up the martial-law authorities.

The Tass attack on Reagan also seemed designed to score propaganda points. It called the president's statements at his Thursday press conference interference in Poland's internal affairs, saying he was "pushing extremists to outrages that already caused bloodshed in Poland."

Tass contrasted Reagan's stated concern for "the rights of free trade unions" with his handling of the recent U.S. air controllers' strike. It said his "assertions sound hypocritical," since he "quite recently crushed the U.S. air controllers' union for participation in a strike and dismissed more than 11,000 air traffic controllers without the right to work in this field in the future."

Some sources here suggested that Moscow's polemics with Washington also may be designed to deflect attention from the Polish situation.

Today, Tass reported the clashes between police and strikers in Gdansk and near Katowice Wednesday in which seven persons were killed and hundreds injured.

More extensive reports from Poland, including frequent mention of difficulties and unrest, were interpreted by Western analysts here as reflecting Soviet confidence that at the moment martial-law authorities had the country under control.

Moreover, for the first time since the outbreak of Polish unrest nearly 17 months ago, virtually all Soviet dispatches were based on the PAP reports and contained few Soviet comments.

Western diplomats said that Moscow seems to have achieved its initial objective. It has secured the crackdown on Solidarity it wanted without direct Soviet involvement.