By Charlie Devereux

MADRID (Reuters) -Spanish Prime Minister Pedro Sanchez's attempt to draw a line under a corruption scandal was thrown into disarray on Saturday as one of the officials he was set to name in a shake-up of his Socialist party resigned over sexual harassment allegations.

Francisco Salazar offered his resignation as a deputy in the organization's secretariat and asked for the allegations to be investigated, the Socialist party (PSOE) said in a statement.

The PSOE said it would begin an investigation immediately, adding that no allegations had been made through it usual channels.

Online left-wing news website elDiario.es quoted a PSOE employee who accused Salazar of making obscene comments about her clothes and body, invitations to dine alone with him and offers to sleep at his home while working in a role junior to him at Moncloa Palace, the prime minister's official residence.

Reuters was not immediately able to contact Salazar for comment.

The scandal involving Salazar came just as Sanchez was scheduled to speak at the PSOE's headquarters in Madrid, where he was due to announce measures to assuage members of his party concerned about the damage to its reputation and its ability to survive.

On Monday, a Supreme Court judge ordered that former PSOE official Santos Cerdan be held in pre-trial detention after he was accused of orchestrating kickbacks in exchange for awarding public works contracts.

Cerdan denies the allegations, which are part of a wider corruption inquiry threatening to destabilise Sanchez's government.

The minority coalition led by the Socialists relies on a loose alliance of nationalist and far-left parties to pass legislation. Until now, those allies have said they do not plan to support the conservative People's Party's call for a no-confidence vote that would precipitate an election.

Senior party figures arriving at the PSOE headquarters were met with boos from protesters gathered across the road and were forced to raise their voices when declaring their support for Sanchez as the crowd chanted "out!, out!"

While some said they were confident that the measures Sanchez was set to announce would defuse the scandal, others appeared more sceptical.

Castile-La Mancha Governor Emiliano Garcia-Page described the scandal as one of the most serious in the half century since the restoration of democracy in Spain following the death of dictator Francisco Franco.

"The leadership needs to understand that if it doesn't offer an exit, if it doesn't offer solutions, then it's part of the problem," he said.

(Reporting by Charlie Devereux; Editing by Alex Richardson)