French President Valery Giscard d'Estaing extended his hand yesterday to the defested leftist opposition and indicated a willingness to accept many of the reforms it had demanded in the election campaign that ended Sunday.

In a national radio and television address in which he outlined how he intends to govern during his remaining three years in office, Giscard said it was time achieve "a reasonable cohabitation" between the majority and toe opposition in France.

"The whole country expressed a message to its leaders,"Giscard said, "and that message should be heard, regardless of the outcome of the elections."

Giscard invited the leaders of the opposition to call him to discuss" the wrongs that have been committeed on both dsides." He announced that he is planning talks today and tomorrow with the leaders of the two main noncommunist trade unions.

Contrary to some expectations, Giscard refrained last night from designating a new prime minister to replace Raymond Barre, a symbol of fiscal soundness.

One shrewd businessman said, however, that "the fact that the franc is going down again after rising right after the election means that someone knows something. I'll bet Barre is out."

The main candidates mentioned to replace Barre now or later are Justice Minster Alain Peyrefitte, a liberal Gaulisst, and Health Minister Simone Veil, whom the opinion polis show to be the most popular politician in France.

"There are choices of persons that can symbolize openess." said Giscard, speaking of the new government to which, he said, he would give the task of preparing the way for a broad national union."

Giscard's appeal was in stark contrast to the tough line being taken against the opposition by Gaullist Party leader Jacques Chirac.

Behind the debate inside the government majority over how to deal with the half of the French voters who were disappointed by the outcome of the legislative elections, Giscard d'Estating and Chirac are already setting out the battle lines for the next electoral test - the 1981 presidential election. Their behavior makes clear that they are preparing to run against each other.

Although the two sides were seperated by only about 1 percent of the vote on the two successive election Sundays, the majority was reelected with 291 National Assembly seats to 200 for the leftist oppition.

For the first time, Giscard's supporters won almost as many sears as the Gaullist - 137 against 148 Gaullists.

The Gaullist leader warned, however, that he would try to woo into Gaullist ranks as many as possibly of the deputies elected under the Giscardist banner.

Giscard in effect gabe notice last night he intends to try to push through a number of social reforms that Chirac thwarted whrn he served as Giscard's first prime minister.

Giscard's appeal to the left seemed meant to warn Chirac that the president could always seek support from moderate elements of the left, especially among the Socialists, if Chirac tries to frustrate his policies again.

Chirac has served notice that his group will reject a number of favorite Giscardist measure and introduce economic and social legislation of its own.

Former prime minister Michel Debre, widely regarded as a top Gaullist spokesman, said in a front-page article in the respected daily Le Monde that he rejects all the talk about generosity toward the opposition since they would hardly have paid any attention to the opinions of their opponents if they had won.

Debre said that the top priority must be given not to social measures but to a strong currency, the search for independent national energy sources and "a policy warnly favorably to savings and productive investment."

Elections, said Giscard, do not have the purpose of "distinguishing between good and bad French-men . . . Those who voted Socialist Sunday and those who voted Communist are Frenchmen and Frenchwomen like the others, equal members of the national community."

Giscard said he would announce his choice of prime minister after meeting political, union and business leaders. He said he would be asking those who call on him the question, "What in your opinion are the problems that Frence should solve first?"

He said that he has already concluded that France must continue to put its economy back on the rails, including attempts to deal with unemployment, to reduce social inequalities with regular increases in the real pruchasing power of the workers, and to cut bureaucratic red tape and give more power to local governments.

"I am counting," he said, "on the majority, on all of the majority, to support heartily and ardently the policy of renewal that will answer the country's expectations."

If that, he concluded, there will be "aye long springtime for France."

In what is seen as an immediate test of strength inside the majority , former Gaullist prime minister Jacques Chaban-Delmas announced yesterday that he will run against National Assembly President Edgar Faure, who is standing for reelection. Faure has allied himself with Chirac. Chaban advocates a government program similar to Giscard's.