France's Socialist government announced sweeping changes in the country's electoral system today in a move that is likely to bolster President Francois Mitterrand's chances of completing his seven-year term of office.

After months of speculation about the government's intentions, it was decided at a Cabinet meeting chaired by Mitterrand today to replace the present winner-take-all constituency system with proportional representation.

Legislation detailing the new voting regulations is to be submitted shortly to the National Assembly, where Mitterrand's Socialist Party has a comfortable majority, and the new system is to be introduced in time for crucial legislative elections in early 1986.

Political analysts here had regarded electoral reform, which was part of the Socialist Party's platform in 1981, as a trump card in Mitterrand's hand as he enters a high-risk period of his presidency.

The slump in popular support for the Socialists over the last two years following the government's adoption of stringent economic austerity measures has raised the possibility of a potentially dangerous confrontation between a left-wing president and the National Assembly if it is dominated by the right after the 1986 elections, as is widely assumed under the present electoral system.

The prospect for such a clash raises much greater alarm in France than in the United States where it is quite common for different political parties to control the Congress and the White House. All French heads of state since Charles de Gaulle ended the "revolving door" system of the Fourth French Republic in 1958 have been able to rely on an absolute majority within the National Assembly.

Mitterrand's term of office does not expire until 1988. But under the division of powers in the French constitution, a hostile National Assembly could compel the president by law to take actions politically repugnant to him, and many French politicians have predicted that rather than face such a situation, Mitterrand would resign.

Analysts predict that the electoral reforms could have far-reaching consequences for French politics by making it more difficult for any one party to secure a majority in the National Assembly, France's main legislative body. In theory, the new system should allow the president greater room for political maneuvering in putting together a new coalition government capable of winning a vote of confidence in the assembly.

Under the new system, the National Assembly will be increased by 86 members to a total of 577. Presidential elections will be unaffected.

Spokesmen for the mainstream opposition parties were unanimous in denouncing the government's decision as an electoral ploy by Mitterrand designed to rob the right of outright victory in next year's elections.

A joint declaration by the neo-Gaullist Rally for the Republic Party, headed by Paris Mayor Jacques Chirac, and former president Valery Giscard d'Estaing's Union for French Democracy accused Mitterrand of choosing "his personal interest above the national interest." It described the new voting system as "incompatible with the institutions of the Fifth Republic," inaugurated by de Gaulle in 1958, and predicted that it would lead to "government paralysis."

Prime Minister Laurent Fabius said on television tonight that proportional representation would introduce "more justice" into the electoral system by making each vote more equal.

The voting procedures announced today are similar to those used under the Fourth Republic between the end of World War II and 1958. The political parties will draw up lists of candidates for each of France's 100 departments, the basic local administrative districts, and seats in the National Assembly will be distributed in rough proportion to the number of votes received by each list.

Only parties that receive at least 5 percent of the votes in each local contest will be eligible for representation in the assembly.

Under the present system, elections are held in two rounds. The first round is frequently viewed as a kind of popularity contest, with the second round usually a straight left-right clash between the two leading candidates.

By penalizing the smaller parties, the present system has produced a succession of clear-cut results for the National Assembly and ended the period of unstable coalition governments that characterized the Fourth Republic.

Computer projections based on present voting patterns indicate that the parties that stand to gain most under the proposed system are the Socialists, the Communists and the extreme right-wing National Front. Both the Rally for the Republic Party and the Union for French Democracy would lose ground, making it less likely that they can achieve a joint overall majority in the National Assembly in next year's election.

The change in electoral law has been opposed by some members of Mitterrand's Socialist Party, including Agriculture Minister Michel Rocard, who has frequently been mentioned as a possible presidential candidate. In a radio interview this week, Rocard described proportional representation as "a defeatist choice" and "a major step backward."

Rocard told Reuter news service in a telephone interview early Thursday morning that he had submitted his resignation to Mitterrand. He said the decision was linked to the Cabinet meeting, but he declined to elaborate.

Despite this internal opposition, Mitterrand's advisers say they are confident they will have little difficulty pushing the electoral reform plans through the National Assembly given the comfortable Socialist majority.

Socialist Party leaders have rejected opposition criticism of the timing by pointing out that previous right-wing governments have given much shorter notice of changes in the voting system before important elections. They also have cited previous statements by right-wing politicians endorsing the principle of proportional representation.