Voters today handed France's Socialist-led government a significant setback in local elections, confirming conservative control of county councils that for the first time in French history have been granted substantial financial and political power.

Less than 10 months after the Socialist Party triumphed in presidential and legislative elections, their leaders readily conceded today that the electorate has given them a warning by voting conservative and inflicting a humiliating defeat on the Communist junior partners in the government.

Confirming the trend of last Sunday's first-round vote--in which only candidates with outright majorities were elected in an often crowded field--today's runoff results showed that the Socialists remain France's largest party, but only barely.

With all 2,029 seats in mainland France accounted for, the Socialist-Communist coalition--which had hoped to take the majority of the 95 departments, or counties--controlled only 34. The opposition won control of 57 councils, an increase of six, and four others remained undetermined.

The biggest winner today was the neo-Gaullist Rally for the Republic, led by Paris Mayor Jacques Chirac, which captured 332 council seats, compared to 191 in 1979.

Its former coalition partner, the Union for French Democracy, once headed by former president Valery Giscard d'Estaing, took 446 seats, up from 401 three years ago.

The Socialists won 507 council seats, a drop of seven, while the Communists fell from 242 seats to 194. The remainder of the seats were won by independents.

In popular votes, the left slightly outstripped the conservatives' combined opposition, the Interior Ministry said.

Thanks to a Socialist reform, the councils are to be given sweeping financial and political autonomy from the traditionally Paris-controlled central government.

Despite the results, the Socialists still enjoy an outright majority in the National Assembly for the next four years. Socialist President Francois Mitterrand's term expires in 1988.

The Communists' showing suggested that even loyalists were deserting a party beset by inner difficulities that martial law in Poland has worsened.

It was uncertain whether the results would set off a new rash of speculation against the franc, which registered its worst showing against major currencies Thursday before rallying slightly Friday.

With Socialist ministers decrying what they said was a politically motivated run on the franc, French press reports said the Bank of France had spent as much as $3 billion to defend the currency since the beginning of March.

Jean Lecanuet, president of the Union for French Democracy, and other opposition leaders said voters had rejected the "profound changes" they had approved last year. But other analysts remained convinced the setback was meant as a warning to a government that too often has appeared to be floundering and inconsistent in a country that likes tight leadership.