Socialist President Francois Mitterrand today laid out the ground rules for the power-sharing arrangement between him and the country's new right-wing government, telling the National Assembly that he would act to preserve their constitutional prerogatives from possible governmental encroachment.

The president's defense of the rights of parliament came in a message to the assembly on the functioning of what has become known here as "cohabitation." This is a French term used to describe the division of executive power between political opponents following the narrow right-wing victory in last month's parliamentary election.

Mitterrand said that the election created an unprecedented situation in the 28-year history of France's Fifth Republic -- the first time that parliament and government were controlled by political parties opposed to the president.

The president, whose term does not expire until 1988, insisted that France's political institutions were strong enough to survive the challenge as long as everyone respected "the Constitution, nothing but the Constitution, the whole Constitution."

In keeping with tradition, all the deputies stood in silence as Mitterrand's message was read by the president of the assembly, Jacques Chaban-Delmas, a former conservative prime minister. By addressing the assembly in this way, the president was using a technique employed by the founder of the Fifth Republic, Gen. Charles de Gaulle, for some of his most startling pronouncements.

Political commentators here interpreted Mitterrand's intervention as an attempt to show that he accepts the outcome of the election but will not allow the new conservative government complete freedom of action. The government's program is to be formally unveiled in a speech to the assembly, the main chamber of parliament, by conservative Prime Minister Jacques Chirac on Wednesday.

The most controversial passage in Mitterrand's address came when he tried to limit the government's right to bypass normal parliamentary procedures by passing legislation by decree. With a slim two-seat majority in the assembly, the government had been hoping to use decrees to speed the passage of new laws undoing the 1981 nationalization of banks and large industrial groups by the Socialists and revising the electoral system.

According to the Constitution, the government can rule by decree for a limited period once it has secured the agreement of parliament. But the decrees do not become law until they have been countersigned by the president.

While acknowledging that the previous Socialist government had ruled by decree, Mitterrand said that decrees should be few in number and well-defined in order to respect the rights of parliament. He noted that the Socialists had not used decrees to enact major reforms such as nationalizations and the codification of workers' rights.

Political analysts noted that the president stopped short of saying exactly which decrees he would veto. In private, he has made clear that he will defend what he calls the "social gains" registered under the period of left-wing rule, including the restriction on the right of managers to dismiss workers.

An early test of the government's ability to rule by decree is likely to come with plans for "denationalization" and electoral reform that will be unveiled over the next few weeks.

Recent public opinion polls have indicated that several left-wing reforms that the right has pledged to dismantle, notably administrative restrictions on dismissals and a wealth tax, are supported by a majority of French citizens.