WARSAW (Reuters) - European Council President Donald Tusk testified for eight hours on Wednesday in a Polish intelligence probe by Warsaw’s right-wing government that he described as a smear campaign to discredit him.

Donald Tusk, the President of the European Council, leaves after meeting Britain's Prime Minister, Theresa May inside 10 Downing Street, in central London, Britain April 6, 2017. REUTERS/Hannah McKay

Tusk was called as a witness in an investigation into former heads of military counter-intelligence (SKW) suspected of cooperating with foreign intelligence services without his official permission.

A former SKW head has said that Tusk, who was Poland’s prime minister at the time and thus ultimately responsible for the intelligence service, was fully aware of secret services’ cooperation with Russia and had authorized it.

The 59-year old EU official is the ex-leader of Poland’s Civic Platform (PO), now the largest opposition party, and an arch-rival of Jaroslaw Kaczynski, head of the ruling eurosceptic Law and Justice (PiS) party.

The prosecutors who summoned him to testify are under the direct control of the PiS-run government.

“I have no doubt that this is part of a political smear campaign. Its authors do not really hide it,” Tusk told local media before the hearing in the prosecutor’s office.

Tusk said afterwards that he was barred by law from providing any details about the probe. “I have come here because of respect for the Polish state,” he said.

He said he would invoke his immunity as European Council president if he felt prosecutors were trying to make it impossible for him to perform his duties in Brussels. They could do that, for example, by repeatedly summoning him to testify in Warsaw.

“I hope it will not come to this, but if it does I will not hesitate,” he said.

Earlier, Tusk was greeted by hundreds of supporters at Warsaw’s central railway station chanting “Free, European Poland”.

‘MAY FACE CHARGES’

Kaczynski, 67, has said Tusk should not be reappointed to chair summits of EU leaders because he may face charges in Poland related to a 2010 plane crash that killed the country’s then president - Kaczynski’s twin brother Lech - or other accusations about a Ponzi scheme.

Polish Prime Minister Beata Szydlo was the only EU leader to object to Tusk’s reappointment at a summit on the issue. Although Kaczynski holds no government post, he is seen as Poland’s main decision-maker.

Justice Minister and Prosecutor General Zbigniew Ziobro said Poland’s “establishment wants to portray Donald Tusk as a victim and a mistreated politician.”

“But Donald Tusk enjoys the benefits of Polish citizenship, while also having responsibilities to fulfil,” Ziobro told state television.

The PiS party, which defeated Tusk’s PO in a 2015 election, has been accused by the European Commission of undermining democracy with its overhaul of the constitutional court.

Tusk’s lawyer Roman Giertych said earlier this month that prosecutors had threatened twice to use physical force against the former prime minister to bring him to the hearing despite his EU immunity.

Earlier in March, Defence Minister Antoni Macierewicz, a close ally of Kaczynski, accused Tusk of actions amounting to diplomatic treason, saying he worked with Russia to harm Polish interests in connection with the 2010 plane crash over Russia’s Smolensk.