Socialist and Communist candidates working together captured at least 25 important city halls held by the governing coalition in the first round of nationwide municipal elections today, partial returns showing.

The two allied leftist parties, which have strong hopes of coming to power nationally within the next year, also registered strong advances in contests that will have to be decided by run-offs next Sunday.

In the hotly contested race for mayor of Paris, Gaullist leader Jacques Chirac took a clear lead in the balloting over Michael d'Ornano, the candidate supported by President Valery Giscard d'Estaing.

Chirac's apparent victory over d'Ornano holds national significance. The Gaullist leader quit as GIscard's prime minister last August partly because of their strong differences over a stratefy for confronting what appears to be a growing political threat from the left.

Since leaving the government, Chirac has been trying to refashion the Gaullist party into a more traditional right-wing political force that would confront the Socialists and Communists directly rather than trying to woo voters with gradual reforms as Giscard has.

Electing its first mayor in a century, Paris was one of the few places in which the leftist alliance showed no signs of new strength and Chirac appeared to be well positioned to win next week's runoff against a joing Communist-Socialist ticket.

Ecologist candidates captured a highly respectable 10 per cent on the average in the French capital, which has been run by government-appointed administartors since the Commune of 1871.

Overall, computer projections run by French television indicated that the Communist and Socialists would gather about 52 per cent of the votes cast across the country.

The impact of local issues and personalities detracts from the significance of that estimate. But when compared to their showing in France's last municipal elections in 1971, when the two parties gathered only 15 per cent of the city council posts at stake in the voting, the figure shows the size of the gains registered since the 1972, when the Communists and Socialists agreed to a joint electoral program.

Postelection comments by political leaders here also identified the new lestists surge as the most important outcome of today's vote.

Communist Party leader Georges Marchais hailed the early results as showing "the deep discontent in the country with the government's policies. It shows that the electorate clearly approves of the unity of the left and that positive results can be obtained when Communist candidates head the ticket."

Chirac kept up his fire on Giscard's leadership by asserting that the new "thrust by the left is due not to its strength, but to the weakness within the majority," as the parliamentary coalition of the Caullists, Giscard's Republican-Independents, reformists and centrists is known.

France faces "a period of tension and confrontation untilthe parliamentary elections," former Foreign Minister Michel Jobert said as the results came in. A ticket backed by Jobert in Paris failed to win any significant support.

Parliamentary elections are scheduled for 1978, but the results of the municipal elections revived speculation here that Giscard may be forced to advance the voting for the National Assembly to this year to end a climate of uncertainty that is inhibiting investment and economic recovery.

"Today's results and those of next Sunday will force the president to reflect on what will be the best moment to admit that his forces have lost, Socialist Party leader Francois Mitterand told a radio interviewer. Giscard defeated Mitterand for the presidency in 1974 by less than one percentage point in the popular vote.

A larger than usual turnout to pick 36,000 municipal boards, which then elect mayors from their ranks, also indicated the importance of today's vote. About 70 per cent of the 33 million registered voters went to the polls, about the proportion usually accorded legislative elections.

The first major shocks for the now-feuding coalition of Gaullists and Giscardians came in the champagne capital of Reims, where a Communist-led ticket ended 18 years of Gaullist domination of city hall. Angers, Brest, Chartres and Cherbourg were among the other towns of more than 30,000 population that went over to the Communist-Socialist side on the first round, when an absolute majority was required for victory.

The left, which lost only three cities to the majority today is likely to emerge from next Sunday's runoffs with control of more than half of the 221 French cities that have more than 30,000 inhabitants, today's results indicated.

In Paris, city council candidates committed to vote for Chirac piled up convincing margins in the traditional Gaullist areas of the affluent center city and then went on to take away two swing districts that might have been expected to show more sympathy for Giscard's reform programs!

The d'Ornano ticket headed by Culture Minister Francoise Giroud in the 15th Arrondissement, the most populated administrative district in Paris, was soundly defeated by Chirac's forces, who also won the Left Bank's properous 3th Arrondissement. Chirac established clear leads in 11 of the city's 18 voting districts.

Five hours after the Paris polls had closed, d'Ornano continued to refuse to drop out of the race and throw his support to Chirac, although his lists were making good showings only in three of the city's 18 voting districts. d'Ornano ran strong in the silk-stocking 16th Arrondissement, and in the socially and economically mixed 18th Arrondissement, where d'Ornano sought a city council seat.

Computer projections of the Paris vote gave Chirac 28 per cent to d'Ornano's 20 per cent.

In Marseilles, one of the few cities where the Communists and Socialists opposed each other rather than selecting a joint ticket, Socialist Mayor Gaston Deffere finished well ahead of his Communist rivals and appeared to be certain of victory next week in the runoff. In Toulon, however, the Communists ran well ahead of a Socialist ticket.

Former Gaullist Prime Minister Jacques Chaban Delmas, whom Chirac deserted in 1974 to help Giscard win the presidency, easily won reelection as mayor of Bordeaux and appeared to strengthen his chances for a seat in the new Cabinet Giscard is expected to form after the municipal elections are out of the way.