President Francois Mitterrand's Socialist Party won an overwhelming majority in France's parliamentary elections today with more than 282 seats out of 491.

It was only the second time in this century that a single party has won the majority in the lower house of parliament, the National Assembly. The vote was a resounding confirmation of Mitterrand's May 10 presidential victory over Valery Giscard d'Estaing.

As a result, the French left seems assured of the prospect of governing for a long, uninterrupted period, based on the mandates of a Socialist president for a seven-year term and of a Socialist National Assembly for a five-year term.

The great losers in France's four successive votes in less than two months -- two rounds for the presidency and two for the assembly -- seemed to be Giscard and his party and the Communist Party and its leader and presidential candidate, Georges Marchais.

With only the results from a dozen overseas territories still to be accounted for, the Communists had lost half their assembly seats, dropping from 86 to 43, the direct result of the loss of a quarter of their electorate to the Socialists in both the presidential and parliamentary elections.

The Giscardists fell to 70 seats form 133 in the assembly elected in 1978 and dissolved by Mitterrand to make way for the new elections that took place today and last sunday. The Gaullists slipped from 155 seats to 82.

The results were a victory for Mitterrand's political bet that the French voters would give him a clear majority to carry out his program of limited nationalization in key industries rather than balance his election with a parliament that would serve, in the words of Gaulist Party leader Jacques Chirac, as "a counterweight."

Giscard's closest political lieutenant, former interior minister Michel Poniatowski, won attention with a statement, obviously issued on behlaf of the defeated president, saying, "He will say nothing, but he is the alternative. In him, France has a statesman who is competent, who has the experience of seven years of wise government and who is also 10 years younger than his successor."

Poniatowski said France had "lost a great president" and that this would soon be recognized. The French public soon will see it has been "tricked" as the French franc falls along with the economy and living standards, the added. He predicted devaluation, recession and more unemployment. Socialist only know how to "spend and disorganize an economy," he said."They do not know how to govern widely. . . ."

In contrast to Poniatowski's attack on the Socialists, Guallist leader Chirac said, "We should draw the lessons from events. We will seek the causes in ourselves, and not elsewhere, as others might be tempted to do. We did not manage to convince the French people that we were able to guarantee the changes they expect."

The Socialist Party first secretary, Lionel Jospin, said, "We will prove that we are tolerant and that we believe in give and take. . . . You have placed your confidence in us. I think we will be worthy of that confidence."

Paradoxically, the Socialist majority seemed to make it more likely that the Socialists will try to bring a few Communists into the Cabinet to neutralize potential Communist criticisms from the left. Prime Minister Pierre Mauroy said tonight that negotiations with the Communist participation in the government is "a possibility." Communist leader Marchias reiterated his party's desire to be in the Cabinet.

Deputy Communist leader Charles Fiterman said his party will not ask for a number of Cabinet seats in proportion to the party's actual vote. That seemed to indicate that the Communists have decided to be accommodating in negotiations with the Socialists.

Marchais also indicated that he recognizes his own position as party chief may be at stake. He predicted a broad debate about party strategy and tactics and the responsibilities for its losses. It would not be "tragic" if he were to lose his post, he said.

What was described as an "unofficial" white House reaction to the halving of the number of Communist legislators was read out on French television to the effect that the Reagan administration views the result with "relief" and that it should make Mitterrand's job easier.

A prominent Socialist Cabinet minister noted that the arrival here of Vice President Bush, scheduled for Wednesday, could be misinterpreted as an effort by Washington to exert pressure on Mitterrand not to take in the Communists. b

The Mauroy Cabinet, dominated by moderates, is scheduled to resign immediately, most probably Monday. Mauroy is then expected to be called on to form a new one in which the most prominent figures, such as Finance Minister Jacques Delors, Foreign Minister Claude Cheysson and Interior Minister Gaston Defferre, would be kept on.

The new Cabinet is expected to be put in place this week, perhaps in time for the next regular Cabinet meeting on Wednesday. The new National Assembly is to be convened the following week, on July 2. The initial plans are to keep it in session through the month of July to enact the new Socialist government's first laws and to call a recess for August, the traditional French vacation month.

All 32 current Cabinet ministers who ran for assembly seats were either elected last Sunday with more than half the vote or this Sunday in runoffs.

Runoff winners today included Economic Planning Minister Michel Rocard, Industry Minister Pierre Joxe, National Soidarity Minister Nicole Questiaux, Agriculture Minister Edith Cresson, African Cooperation Minister Jean-Pierre Cot and European Affairs Minister Andre Chandernagor.

Other Socialist leaders elected today include Mitterrand's successor as party first secretary, Lionel Jospin; party spokesman Claude Estier; women's liberation leader Gisele Halimi; civil liberties lawyer Roland Dumas, and Mitterrnand's son, Gilbert.

Marchais was one of the few well-known Communists to retain his seat.

In the old majority, winners today included former prime minister Michael Debre, former environment minister Michel d'Orano, former defense minister Robert Galley and fomer youth and sports minister Jean-Pierre Soisson.

The latter two had been expected to lose on the basis of last week's results. But enough voters who had stayed home on the first round voted for them to reverse the tide.

A record 30 percent of the electorate did not vote last week. The stay-at-homes were reduced to 24 percent this week, but the new voters voted much like the old ones.