MADRID (Reuters) -Spanish police entered the headquarters of the ruling Socialist Party on Friday to copy the emails of ex-senior party official Santos Cerdan, whose resignation amid a widening graft probe last week triggered a severe political crisis.

Judge Leopoldo Puente, who is investigating allegations against Cerdan and former Transport Minister Jose Luis Abalos, had ordered that the party let the plainclothes officers access its premises, although the Guardia Civil police and government officials said the move did not amount to a raid or searches.

Government spokesperson Pilar Alegria and Transport Minister Oscar Puente also confirmed a similar visit to the ministry's roads department.

The resignation of Cerdan, a close ally of Prime Minister Pedro Sanchez, on June 12, and subsequent revelations of potential misconduct, have destabilised the minority governing coalition.

Sanchez has apologised publicly but rejected the opposition's calls for his resignation. In a tense grilling in parliament on Wednesday, he declined to say whether his name could crop up in the investigation.

The Cerdan case is the latest and most severe of a series of scandals that has left Sanchez having to fend off calls from opponents for snap elections.

Cerdan resigned from his seat in parliament and his post in the Socialist Party after a police report was sent to the judge.

The report, seen by Reuters, provided transcripts of recordings of Cerdan, Abalos and the latter's former assistant Koldo Garcia discussing alleged kickbacks.

Cerdan "appeared to be the person in charge of taking those alleged payments," the police wrote in the report. He has publicly denied any wrongdoing.

The judge also asked the police to pore over bank accounts held by Cerdan, companies which allegedly paid bribes and five entrepreneurs. Around 500 bank accounts will be analysed, according to the writ.

He also ordered state-owned railway infrastructure operator Adif and the transport ministry's roads department to hand over files over public works that were questioned in a preliminary police report.

Abalos and Garcia will testify before the judge next week, while Cerdan will appear on June 30.

(Reporting by Emma Pinedo and Inti Landauro, editing by Andrei Khalip, Alexandra Hudson)