Segolene Royal used a word notoriously linked to her rival

She published a list of her assets and denied she and her partner had tried to limit their exposure to wealth tax.

Just days after Mr Sarkozy was confirmed as the candidate for the ruling centre-right party, she accused his camp of a "thuggish" campaign.

Correspondents say it marks the opening salvo in the presidential race.

"The campaign launched by the UMP is a dishonest campaign... a campaign of thugs," she said.

She used the French word "racaille" to describe the tactics - an evocative term that has been translated as "thugs", "filth" or even "scum".

It is the same word Interior Minister Sarkozy once used to describe mainly dark-skinned rioters in the suburbs, causing a furore.

Socialists worried

Ms Royal was responding to accusations circulating on the internet that she and her partner, the Socialist Party general secretary Francois Hollande, had taken steps to avoid paying wealth tax on their properties.

She said Mr Sarkozy's entourage was trying to discredit her, but that she would not be intimidated.

The polls suggest Nicolas Sarkozy has landed the first blow

Mr Hollande is taking legal action for defamation against an MP from the governing UMP party over the accusations.

Ms Royal has also been fighting accusations of splits between her own team and the Socialist Party.

Some party members have expressed their disquiet that her campaign is starting slowly, with her manifesto not due until late February or early March. The first round of the election will be held in April.

"We have the impression that there's a vacuum in the campaign," Vincent Peillon, a Socialist MEP, told Le Parisien newspaper.