Nicolas Sarkozy's campaign director for the French election wrote an acerbic letter to his counterpart in the camp of rival Francois Hollande Friday, asking that the Socialist Party keep control of militants who mobbed the president this week.

Sarkozy was jostled by a crowd of left-wing militants and Basque activists, as he visited the southwestern city of Bayonne in France's Basque country, by the Spanish border, Thursday.

Riot police had to be brought in as militants whistled at Sarkozy, threw scrunched-up political tracts at him and lobbed eggs at the window of a bar where he sat out the protest.

The scuffle marked an escalation in tensions in an election race already characterised by jibes on both sides as Sarkozy battles to defend his turf from Hollande, who has a clear lead in opinion polls.

Insults against a candidate will simply not be tolerated. These outrages serve neither the campaign nor democracy, the ruling conservative party's Guillaume Lambert wrote to Hollande's campaign chief Pierre Moscovici, a former European affairs minister.

For that reason, I would ask you to be vigilant in future to ensure that Socialist Party's militants and representatives do not receive orders, as was the case in Bayonne, to take part in this kind of anti-democratic practise, seeking to forcefully disrupt Nicolas Sarkozy's electoral campaign.

Sarkozy, who launched a high-energy campaign in mid-February that has put Hollande on a defensive footing with his month-old election program, called the protesters louts as he arrived in Brussels later for a European summit.

Addressing a campaign rally in the city of Lyon Thursday evening, Hollande did not allude directly to the incident, but said there was no excuse for verbal or physical violence.

Out with reporters in Paris Friday, Hollande -- who has made it a feature of his campaign to always refer to Sarkozy as the outgoing candidate rather than by name -- said: I will not take part in this verbal oneupmanship.

The public expects a dignified campaign. The outgoing candidate, who is already ending a presidency marked by excess, should not run an over-the-top campaign of excess, he said.

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Election poll graphic: http://r.reuters.com/was36s

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MILLIONAIRES TAX

Sarkzoy made a blazing start to his campaign that saw him eat into Hollande's lead in what is now a clear two-horse race, but more recent opinion polls indicate that spurt may be waning.

A daily survey by pollster Ifop based on a constant sample of people found Sarkozy slipped 1 point and Hollande gained 1 point in its Thursday evening result for the April 22 first-round vote to stand at 25.5 percent and 29 percent respectively. Most polls give Hollande a double-digit lead for a May 6 runoff.

Far-right leader Marine Le Pen, ranked third in first-round polls with less than 20 percent, is still 48 signatures short of the 500 mayoral signatures all presidential hopefuls require to register as candidates by a March 16 deadline.

With Sarkozy unveiling his ideas week-by-week, Hollande sought to seize back the limelight this week by proposing a 75 percent tax rate on earnings above a million euros, which would nearly double the country's current upper tax band.

The measure would affect only a few thousand people and raise a largely symbolic 200 to 300 million euros according to Hollande, but the head of France's football league, Frederic Thiriez, has warned that it could destroy French football.

Socialist Party sources say the tax idea was a last-minute one by Hollande, who had said during a debate with news website Mediapart on January 28 that imposing hyper-taxes on the very rich often backfired as people tended to move their tax base abroad.

A punitative tax on a tiny fraction of taxpayers would not produce much revenue, he said.

(Additional reporting by Elizabeth Pineau and Patrick Vignal; Editing by Daniel Flynn)