French President Valley Giscard d'Estaing practically conceded last night that the leftist opposition will win the first round of today's national legislative elections, but he appealed to Frenchmen to reverse themselves in the runoff elections next Sunday.

"In the first round, you will express your preferences, and I know, probably your irritations and discontents," Giscard said in a solemn television address only 12 hours before the opening of the polls.

But in the runoffs, Giscard said, "you will choose, not in terms of preference or discontent, but in terms of reason."

The president's approach was clearly a reflection of the government's own public opinion polls that reportedly showed a heavy trend to the Socialists and Communists on the eve of today's balloting.

French voters in the past have reversed themselves from one Sunday to the next, but this time the gap between the two camps may be too wide to be bridged.

Despite his apparently resigned acceptance of the left's seemingly inevitable victory today, Giscard presented the specter of economic collapse and of domination by West Germany if the French voters do not make what he called "the good choice."

He stopped short of the cataclysmic style of his predecessors, Charles de Gaulle and George Pompidou, who habitually warned voters that the choice was between them or "chaos."

Giscard also avoided explicitly saying whom to vote for but he left no doubt that he was appealing for the present government majority.

The enactment of the left's election promises, he said, would mean a new crisis for a "still very fragile" French economy. It would mean inflation, more unimployment and higher taxes, he said.

The result, he said, would be a return to the days before President de Gaulle when France was reduced to a humiliating quest for foreign loans.

French economic weakiness, he warned, would immediately make "our powerful partner" West Germany, economically and monetarily preponderant. "And which of us can resign himself to that?" That appeal was apparently designed to arouse traditional French fears of Germany that are as prevalent on the left as on the right.

Although publication of voter preference polls is banned during the week preceding today's vote, one widely respected polling organization is understood to have found recently that the lead of the leftist opposition has widened to 10 percentage points, up from the 6-to 8 point lead of previous weeks.

"The campign has been somber, the atmosphere has not been gay, even though we are going to win," complained a Socialist leader.

That view is general, and it is attributed to the Communist attempt to overtake the Socialist lead inside the lefist camp, and, if possible, to provoke splits in the Socialist Party.

The Communist attacks on the alleged rightist tendencies of Socialist leader Francois Mitterrand have inhibited the enthusiasm of Communist voters for a leftist victory and raised questions among the Socialists about what king of partners the Communist would be in government.

If the left victory really turns out to be as strong as it thought, it would be difficult for the Communists to carry out threats to sabotage the outcome in the runoff elections March 19.

A well-informed ex-Communist said that as recently as a month ago a party chief told a meeting of party officials that a government of the right would be preferable to one run by Social Democrats.

The latest unpublished polls reportedly show that the leftist gains are almost all for the Socialist Party.

Communist leader Georges Marchais has been saying that when the leftist leaders meet tomorrow to discuss the results of today's vote, reaching an agreement to withdraw from each other's candidates who are leading first-round voting should be no problem. Some analysts say that Marchais could be laying the groundwork for blaming the Socialists if the negotiations fall apart. The deadline for candidates to withdraw is midnight Tuesday.

Marchais served notice Friday night that one of his key demands tomorrow will be that there be Communists in the Cabinet in proportion to their electoral strength, meaning six or eight posts in a cabinet of 20 to 25.

Marchais also said he would demand equal rights to key ministries - a phrase that in French political paralance usually means defense, interior (police), finances and foreing affairs.

All associate of mitterrand's said that the Socialist leader broke off Cabinet discussions with the Communists when he realized that they wanted to give the Socialists all the unpopular tasks and take the attractive ones for themselves.

As examples he rited Communist proposals to split the finance ministry between economic planning and taxation, leaving tax-collecting to the Socialists. The Communists also proposed splitting interior between the police and relations with local governments, leaving the Socialists the police.

Another reported Communist idea was to split the Defense Ministry between the armed forces and the large government armaments industry, taking over arms production and the 75,000 workers in the industry for themselves.

The polls seem to have laid to rest a spectre that was haunting the left - that it could win a clear majority of the votes and still not win a majority of the 491 seats in the National Assembly.

This is possible because the French electoral map is stacked in favor of rural and middle-class districts. The largest district in France is a Communist one in the Paris region with 318,000 inhabitants. The smallest district is a middle-class one in the Paris city limits with 56,000 inhabitants and a Gaullist national assemblyman. Of the ten most populous districts, eight have leftist assemblymen and large proportions of the leftist votes in those districts are "wasted."

It is generally calculated that the left needs 52 percent of the vote to get an assembly majority. In 1968, the pro-government parties got three-quarters of the assembly seats with 46.4 percent of the vote.