* Marine Le Pen to spoil ballot, no advice for supporters
* Le Pen eyes June parliamentary elections as runoff looms
* Sarkozy says centrist unlikely as future prime minister
* Sarkozy, trade unions, Socialists vie for May Day crowds
PARIS, May 1 (Reuters) - French far-right leader Marine Le Pen delivered a further blow on Tuesday to President Nicolas Sarkozy's re-election hopes by refusing to endorse him and telling her six million supporters to make their own choice in Sunday's ballot.
Conservative Sarkozy, who faces off with Socialist Francois Hollande on May 6, needs most of the 18 percent of voters who backed National Front leader Le Pen last week to support him in the runoff if he is to overtake first-round winner Hollande.
But Le Pen, who came third on April 22 with a score that eclipsed her father's record at the head of the populist protest movement, told a rally in Paris on Tuesday that she personally would spoil her ballot paper in the second round by choosing to vote for neither of the two remaining contenders.
"I will not grant my trust, or a mandate, to these two candidates," she told supporters at an annual commemoration of Joan of Arc, the national saint her group favours to the May Day celebrations held by international labour and leftist parties.
"On Sunday, I will cast a blank ballot," she said, while stopping short of urging followers to do likewise. "I have made my choice," she added. "Each of you will make yours."
Sarkozy fired back at his own outdoor rally, on a holiday that turned the streets of Paris into a peaceful political battleground: "I will fight to the last second of the last minute because I love France," he told supporters.
"Nicolas, president!" they chanted back on a square across the Seine from the Eiffel Tower. "We're going to win!"
Hollande has a six- to 10-point opinion poll lead over the president for the May 6 second round, profiting from disillusion with Sarkozy's economic record. Hollande also promises to boost growth and raise funds with a new 75-percent income tax.
Supporters of Sarkozy's UMP party at the rally on Trocadero Square, like 67-year-old Jacqueline Rousseau, fear such policies will hurt France and accuse Hollande of double standards: "If Nicolas Sarkozy doesn't make it, all the rich will leave and we'll be just a country of poor people," said Rousseau.
"Why would they stay with us, to be taxed 75 percent? They're all jealous, but the Socialists eat caviar every day."
Analysts calculate that Sarkozy needs as many as 80 percent of Le Pen's first-round voters behind him to win. But polls indicate only about half of them intend to vote for him, while as many as one in five could back Hollande.
With a parliamentary election to come in June, National Front leaders believe they can break through and win seats in the legislature, especially if a heavy defeat for Sarkozy plunges his centre-right UMP into deeper disarray.
MAY DAY DISPUTES
Sarkozy, 57, is the most unpopular president to seek re-election and first in the 54 years of the present electoral system to fail to finish first in a first-round vote. Economic gloom counts against him, as does a personal style that seems dynamic to supporters but showy and shallow to critics.
Whoever wins on Sunday, investors see a lack of economic growth undermining efforts to cut government deficits just as concerns are resurfacing about the wider euro zone debt crisis. Both candidates have been criticised abroad for lacking big ideas to bolster competitiveness and rein in state spending.
Sarkozy, who used his Labour Day speech to renew his calls for more entrepreneurial spirit in a country where one worker in five is a public employee, has stressed his experience of running the economy during the campaign while Hollande has said Sarkozy must bear the blame him for strained public finances.
For his own Labour Day message, Hollande, 57, paid respects at the tomb of popular Socialist prime minister Pierre Beregovoy - who shot himself on May 1, 1993 - and derided the president's stress on work when unemployment was at a 12-year high.
The two candidates, who will hold a single televised debate on Wednesday, also exchanged barbs over trade union charges that Sarkozy was trying to hijack Labour Day for his campaign. Tens of thousands of unionists marched in Paris and other cities.
Hollande can count on much of the 15.5 percent of the first-round vote that went to leftist candidates to add to his own 28.6 percent score without changing his campaign tactics.
But Sarkozy is tacking with difficulty between ideas that appeal to National Front voters' dislike of immigrants and those which can bolster his support in the liberal centre.
On Tuesday, he appeared to rule out a deal to make losing centrist candidate Francois Bayrou his prime minister. Polls show the 9.1 percent of voters who chose Bayrou in the first round are now split evenly three ways, with a third backing Sarkozy, a third Hollande and a third undecided or not voting.
In a fiery speech to thousands of supporters waving French flags, Le Pen slammed Sarkozy's calls for more border control as pure theatrics and labelled him and Hollande lackeys of the European Central Bank, IMF and European Commission - arguments that have lately bolstered radical parties across Europe.
"The French have started their emancipation," said the 43-year-old who has rejuvenated the party her father founded and who wants France to abandon the euro currency.
Scorning the mainstream parties, the UMP and the Socialist PS as an indistinguishable, pro-EU bloc, she coined an acronym for them and prophesied further gains for a party that hopes to return to parliament for this first time since France briefly experimented with proportional representation in the 1980s.
"The UMPS will not succeed," said Le Pen, whose 83-yaer-old father Jean-Marie stood by her side. "All their efforts cannot stop us growing and cannot block our path to power.
"Our last chance to stop the implosion of France is to bring lawmakers into parliament en masse to stand up to a presidency dominated by Europe and markets."