The French presidential election campaign has sprung to life with President Valery Giscard d'Estaing and his chief opponent, Socialist leader Francois Mitterrand, accusing each other of damaging France's foreign policy interests.

The accusations appeared to shift the focus of the campaign from high unemployment, which Giscard recently called the most important failure of his seven-year term, to foreign policy.

Mitterrand said Monday in his first television appearance of the campaign that a virtual endorsement of Giscard last week by the official Soviet newspaper Pravda was a "payoff" for Giscard's meeting in Warsaw last May with Leonid Brezhnev, the first meeting between Brezhnev and a Western leader after the Soviet invasion of Afghanistan.

The Socialist candidate also said that Giscard had served as Brezhnev's "little mailman" when he informed the Western summit meeting in Venice last year of a message from Brezhnev of a Soviet troop withdrawl from Afghanistan that proved to be misleading. Mitterrand said that he for one would not have "slipped under the table" in his dealings with the Kremlin.

The initial reaction from Giscard's camp was the mild remark from spokesman Jean-Philippe Lecat that Mitterrand had been very good on TV but that Giscard was even better. Apparently under prodding from Giscard, however, there were governmental counterattacks in which Foreign Minister Jean Francois-Poncet spoke of "an insult to France" and Prime Minister Raymond Barre said he was "revolted" by Mitterrand's language with its risk of damaging the image of France in the world.

The counterattack turned what had been a largely low-key campaign for next month's election into a heated exchange.

It came as Mitterrand was making points with moderate anti-Giscardists by saying that he would not take Communists into his cabinet so long as the Communist Party continues to reject the leftist political unity pact it sabotaged, to "play a double game" by helping Giscard and to support the Soviets in Afghanistan and Poland.

Since the Communists do not seem ready to abandon any of that, Mitterrand said, Communist cabinet ministers are unlikely. He appealed to Communist voters over the heads of the party leadership, recalling that in 1944, wartime resistance leader Charles de Gaulle had said he needed everyone's help. "It was the war," said Mitterrand. "Now there is an economic crisis, and I need everyone."

Mitterrand also presented himself as the champion of the small businessman.

Meanwhile, Giscard defended himself from charges today by the satirical weekly Canard Enchaine that his claim to have sold off the diamonds he had received from deposed Central African emperior Bokassa for the benefit of the country's Red Cross were untrue. The paper a facsimile of a telegram from the head of the Central African Red Cross saying that it had never received any contributions from Giscard, adding. "The financial situation of the Central African Red Cross is catastrophic. It can no longer meet its current expenses."

Giscard had said last week in his first detailed account of what happened to the diamonds that they were sold on behalf of Central African charities, notably the Red Cross, "as soon as" Bokassa was overthrown and replaced in September 1979 by President David Dacko.

Today, the Elysee Palace said a check had been sent last month, on Feb. 4 to the Central African government for transmittal to the Red Cross. Dacko today confirmed receipt of a check for $8,000.

The amount involved seemed certain to arouse new controversy. Bokassa told an interviewer in a coming book that he gave Giscard and his wife more than 236 stones of the best quality produced in his country in sizes ranging up to 10 and 20 carats. Giscard had said on TV that the stones he got were nothing but minor ones suitable only for decorating around larger gems.

A single 10-carat stone of the lowest generally acceptable quality would be worth about $7,000 to $8,000 a carat wholesale, said an experienced Paris jeweler. Jewelry sources say that Central Africa produces stones of average to good quality.