President Valery Giscard d'Estaing and Socialist challenger Francois Mitterrand appeared to have come out about even tonight in a two-hour television debate billed in advance as the highpoint of the runoff election for the French presidency.

Giscard sought points by stressing that the Socialist would have no choice but to try to form a parliamentary majority with the Communists, an assertion that Mitterrand apparently found hard to dispute without offending the Communist voters he needs to win in Sunday's balloting.

Mitterrand, in turn, obviously got under Giscard's skin by constantly reading quotations highly critical of the incumbent by Gaullist leader Jacques Chirac, whom the president has been publicly wooing in a series of speeches in the past few days.

An exasperated Giscard finally asked why Mitterrand kept quoting the Gaullist Chirac and never Communist leader Georges Marchais.

While Giscard seemed to come out ahead on debating points, Mitterrand appeared to be not far behind, if not virtually even, which was probably all that he needed not to lose votes.

There will be no further opinion polls under French election rules, but the two most respected pollsters, in a profession that has taken a battering in France during this election, are both understood to be finding Mitterrand ahead in soundings being circulated to newspapers and political organizations.

The debate began with the two candidates arguing over who would have the most trouble governing with their respectively recalcitrant Communist and Gaullist partners. But most of the time was given over to economic and social policy, with each man primarily appealing to narrow interest groups.

The angriest exchange came when Giscard took offense over Mitterrand's suggestion that he would be a better defender of French fishermen in the European Common Market. Relatively little time was spent on foreign policy or other general themes, and the two journalists presiding over the debate asked few sharp questions of either candidate.

One of the two, chosen in bargaining almost down to the wire, political reporter Michele Cotta of the weekly Le Point, complained afterward that the complicated arrangements worked out in a week of negotiations between the two sides had resulted in a hybrid formula that was neither an American-style presidential debate with reporters probing for weak points nor the no-holds-barred direct confrontation in the French manner that Giscard had said he wanted.

The Socialist candidate recalled the bad memories he had of his last televised debate with Giscard when they ran against each other in 1974. He said Giscard had interrupted him 45 times in what was generally considered a largely successful effort to throw the Socialist off his pace.

Mitterrand clearly performed much better this time than last, according to viewers who had watched both debates. "As long as Mitterrand didn't lose," said one moderate voter, "he won."

The Socialist candidate made his points mostly in asides underlining the characteristics of Giscard's personal style that are reputed to have worked against his popularity. When Giscard challenged Mitterrand to say what today's exchange rate was between the French franc and the West German mark, the veteran politician replied, "You're not my professor. You have no right to interrogate me in that tone." Then, Mitterrand went on to give the rate.

He kept referring ironically to Giscard's "professorial" tone, and when Giscard objected that he was being disrespectful, the challenger told him to stop playing president of the republic and to accept the fact that they were two equal candidates.

Mitterrand also stressed the differences in social class between their electorates by such devices as accusing Giscard of wanting Communist voters to be nothing but factory workers and cannon fodder rather than full-fledged citizens. Giscard defended himself by saying, "I never insulted the working class."

At another point, when Giscard tried to get Mitterrand to refer to the unemployed with the official term "job-seekers," the Socialist said, "The word 'unemployed' burns your lips. But people without jobs experience it as unemployment. They feel demoralized, excluded."

Mitterrand reaffirmed that he plans to nationalize private banks and 11 major industrial companies if he is victorious in Sunday's elections, and that other companies apart from those on his "shopping list" could be taken over by the state if they exhibited signs of becoming new centers of capital concentration.

Giscard stressed that Mitterrand's nationalization program would wind up putting half of French industry in state hands, increasing the weight of bureaucracy and reducing the flexibility of the economy.

In a clear appeal to the Chirac electorate, that responded to a Reagan-like appeal for a lighter tax burden on small businesses and independent professionals, the president kept stressing such points in the Socialist program as higher rates of inheritance tax. He tried to demonstrate that the Socialist program would actually result in more unemployment.

But Mitterrand went out of his way to act unimpressed by Giscard's traditional displays of economic and financial expertise. While the Socialist at first let himself be placed on the defensive by having to answer a series of detailed questions, he finally lashed back that there was no need for him to let himself be trapped in Giscard's system since he proposes a completely different economic approach. "If I were to adopt your system," said Mitterrand, "I would fail just as you have failed."

The Socialist kept stressing the government's unkept economic promises, while Giscard said that everything that has been accomplished would be ruined. "There is a taste for change in France," said Giscard. "It is assumed that what is new is better." Giscard said he had made mistakes as president but that he had learned well from them and that he is prepared to preserve France from uncertainty.