(Recasts with opening of police probe)
PARIS, Oct 20 (Reuters) - French prosecutors said on Thursday they had opened a preliminary investigation into unverified media claims that the partner of the Socialist presidential candidate was spied upon.
Earlier , a lawyer for Valerie Trierweiler -- the partner of Francois Hollande who won his party's nomination on Sunday for the election next April -- said she had filed a lawsuit over alleged secret checks that police ran on her.
Lawyer Frederique Giffard said in a statement her lawsuit was filed against "persons unknown", implying all those involved may not yet have been identified.
Paris prosecutors said the probe had been turned over to the internal policing department of the police.
Hollande asked France's conservative government earlier this month to explain a report published in L'Express weekly that police had run secret checks on his partner.
Trierweiler works for a private TV channel and has been with Hollande since he split from fellow Socialist politician Segolene Royale in 2007.
National police headquarters in Paris had denied the report, which cited police sources as saying the spying focused on Trierweiler's life and relationships. Interior Minister Claude Gueant sued L'Express for defamation.
French President Nicolas Sarkozy and his centre-right camp have accused the Left of seeking to exploit recent developments in a number of anti-graft probes where allies of the president have been investigated.
Sarkozy faces a strong challenger in Hollande - popularity polls have consistently indicated he could beat the incumbent in April with ease. (Reporting by Alexandria Sage and Thierry Leveque; Editing by Robert Woodward)
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