PARIS — Partial results in France’s municipal elections yesterday showed President Sarkozy’s conservative party headed toward defeat and losing control of key cities, a new setback for a leader beset by sinking support.

Mr. Sarkozy’s prime minister insisted, however, that the government would forge ahead with a sweeping reform program despite demands for a change from opposition Socialists buoyed by the early returns.

“You can’t change a great country like ours in a few months,” Francois Fillon said. “Tenacity is needed to reform.”

He and his ministers sought to downplay the projected poor showing by candidates from Mr. Sarkozy’s conservative UMP party in a string of large cities — saying not too much should be read into what were local elections for mayors and city councilors.

With 89% of the vote counted, official results showed parties of the left leading just slightly, with 48.7% of the overall vote to 47.6% for the right.

The municipal elections have less to do with party affiliation than other elections, and most voters say they cast their ballots based on local issues such as parks, public transport, and garbage collection.

Yet the race also was a gauge of how the French rate Mr. Sarkozy’s performance. The early results were a reminder of how the 53-year-old president has shrunk in the estimation of many voters since his election in May.

Then, he was welcomed like a rock star by tens of thousands of cheering supporters in Paris. Yesterday, he did not make an appearance, leaving Mr. Fillon to defend his policies in a televised declaration.

Ten months after his election, France’s economy remains sluggish and Mr. Sarkozy has backed off from or toned down some of the sweeping reforms that he promised on the campaign trail. A huge UMP loss yesterday could weaken Mr. Sarkozy’s bid to live up to promises of economic, social, and institutional reforms.

The president has been beset by complaints that he has acted in a manner unbefitting a president with a series of angry public outbursts, a widely publicized divorce, and a quick courtship and marriage to former model and singer Carla Bruni.

A computer engineer from Paris’ Latin quarter, Bruno Marcandella, expressed dissatisfaction with the way Mr. Sarkozy handles “everything — not just his personal life.”

“I’m going to vote because I want my vote to count on a national level,” he said.

Early results showed the opposition had scored several significant victories.