By Gérard Bon and Leigh Thomas
PARIS (Reuters) - President Nicolas Sarkozy overtook Socialist challenger Francois Hollande for the first time on Tuesday in an opinion poll on the first round of France's April-May election after attacking the European Union's trade and immigration policies.
The conservative incumbent was still shown losing to Hollande in a second-round runoff, but by a narrower margin.
Sarkozy's poll boost came as far-right leader Marine Le Pen, ranked third in polls, said she had secured the 500 official sponsors needed to enter the presidential contest.
Candidates have until Friday to obtain the requisite backing of 500 elected officials to compete in the April 22 first round, after which the two frontrunners face off in a May 6 runoff.
A failure by Le Pen to gather enough signatures could have rocked expectations, given her 16 percent support level.
In a morale boost for Sarkozy, who has trailed Hollande in polls for months, an Ifop/Fiducial survey put his first-round score at 28.5 percent, up from 27 percent at the end of February and overtaking Hollande, whose score slipped to 27 percent from 28.5 percent previously.
The poll gave Hollande 54.5 percent of the second-round vote to Sarkozy's 45.5 percent, a narrower lead as Hollande lost two percentage points and Sarkozy gained two.
"I hope the swallow we saw this morning will make the spring," Alain Minc, a longtime advisor to Sarkozy, told Europe 1 radio, citing an ancient Greek proverb popular in France.
The election is a duel between Sarkozy, who promises tighter immigration controls, structural economic reforms and policy referendums, and Hollande, who is running on a tax-and-spend programme while also promising to cut the budget deficit.
The poll was taken after Sarkozy told his biggest rally yet on Sunday that he would erect unilateral barriers to trade and immigration unless the European Union takes tougher stands.
Seeking to breathe new life into his campaign, Sarkozy said Europe should have a law, modeled on the Buy American Act, requiring governments to buy European-made products. He also threatened to pull France out of Europe's Schengen open-borders zone unless progress is made on controlling immigration flows.
Hollande said Sarkozy's move was a sign the incumbent was running out of inspiration. "He will try anything," the Socialist said in a France 3 television interview on Monday.
Election poll graphic: http://r.reuters.com/was36s
ENTER LE PEN
The president, seeking to shed an image of being over-friendly to the rich, said he would crack down on tax exiles by making them declare taxes paid abroad and pay to France the difference with what they would have contributed if still resident.
The proposal followed earlier pledges of curbs on corporate executive pay and a minimum tax on profits at multinationals.
Sarkozy is hampered by the fact he is one of France's least popular presidents ever, with many frustrated at economic gloom and tired of his showy style. An Opinionway survey on Monday put Sarkozy's approval rating down one point at 31 percent, although his disapproval rating dropped two points to 62 percent.
Fifty percent of respondents thought Hollande was running a stronger campaign and 38 percent rated firebrand leftist Jean-Luc Melenchon's most highly. Only 27 percent thought Sarkozy's campaign was best.
Le Pen removed one major uncertainty by disclosing early on Tuesday she had the necessary backing to run.
"I have my 500 signatures and therefore I will be a candidate in the presidential election," she told Reuters.
A charismatic speaker who has won a strong following since taking over the party leadership from her father Jean-Marie Le Pen in January 2011, she no longer seems to have enough support to prevent Hollande and Sarkozy facing each other in the runoff.
Earlier polls had shown her snapping at Sarkozy's heels, reviving memories of the 2002 election when her father sensationally knocked out Socialist Lionel Jospin to reach the runoff against Jacques Chirac, the eventual winner.
As in 2007, Sarkozy is leaning to the right as he tries to woo Le Pen supporters to his side by hammering away on immigration, security and trade protection.
Among other hopefuls, former prime minister Dominique de Villepin said he was still short of the 500 signatures needed from a poll of some 36,000 elected officials.
(Additional reporting by Catherine Bremer; Writing by Brian Love; Editing by Catherine Bremer and Paul Taylor)