PARIS (Reuters) - French President Nicolas Sarkozy's center-right bloc suffered a comprehensive defeat in Sunday's local elections, but managed to cling on to power in one mainland region to avoid a whitewash, exit polls said.

The Socialist party and its allies won some 54 percent of the vote at a national level, to give the left control of at least 20 of the 22 regions up for grabs on the mainland, while the center-right won 36 percent, OpinionWay pollsters said..

The far-right National Front took just 8.7 percent nationwide, according to OpinionWay, but veteran party leader Jean-Marie Le Pen secured backing of almost 25 percent of voters in his region in the far south of France, it predicted.

Although Sunday's result represents a triumph for the Socialists, the fact the center-right appeared set to hold on to the Alsace region in the east was a big relief for Sarkozy, who has come under criticism from his own camp over his policies.

Pollsters said the feeble economy and growing unemployment had taken its toll on the government, with a string of recent controversies, ranging from accusations of nepotism to a debate on national identity, also eroding Sarkozy's popularity.

(Reporting by Crispian Balmer; editing by James Mackenzie)