European Council President Donald Tusk says the questioning leave him in no doubt that he is a political target of the ruling Law and Justice party.
EU President Donald Tusk on Thursday testified in Warsaw as a witness in an investigation into the 2010 jet crash in Russia that killed Poland's then-president and 95 others.
Tusk, who was Poland's prime minister at the time of the crash, told reporters he "had no doubt" he was being targeted by his arch-rival in the current conservative populist government before questioning by Polish prosecutors.
The ruling Law and Justice (PiS) party leader Jaroslaw Kaczynski on Thursday told the Catholic-nationalist Trwam TV station that Tusk "has plenty to fear," adding that the crash probe "is one thing, but there are others."
Kaczynski also unleashed an emotional diatribe in parliament two weeks ago, accusing Tusk's former liberal Civic Platform (PO) party of being responsible for the Smolensk crash that claimed the life of his twin brother, then-president Lech Kaczynski.
"Don't wipe your treacherous mouths with my late brother's name. You destroyed him, you murdered him, you are scoundrels," Jaroslaw Kaczynski told PO deputies.
He has long insisted that the 2010 crash was no accident and accuses Tusk of "moral responsibility" for the death of his brother.
Polish and Russian investigators found that pilot error, bad weather and poor air-traffic control were to blame for the accident.
"What we heard in parliament from Jaroslaw Kaczynski in an outburst of rage and anger points unequivocally to the intentions behind this matter, so I have no doubts here," Tusk told reporters before testifying Thursday.
Grzegorz Schetyna, current head of Poland's Civic Platform (PO) opposition, told Polish media that "Tusk is today a political target for the PiS and the prosecutors who are openly fulfilling a political order."
"This is part of a series of measures targeting him (Tusk). Someone is really afraid of him," Schetyna said.
Prosecutors have said Tusk was due to testify Thursday regarding "public officials who failed to comply with obligations" concerning crash victim autopsies.
Tusk already testified for eight hours in the Polish capital in April as a witness in an investigation of two former military counter-intelligence chiefs accused of overstepping their duties in the initial investigation of the crash.
He was first summoned days after EU leaders gave him another term as president despite strong opposition from Poland.
After taking power in late 2015, the PiS launched a fresh probe into the 2010 crash that also killed a number of senior Polish officials and a commission of enquiry it controls suggested in April that an explosion likely caused the aircraft to break up in the air.
The crash occurred as the presidential delegation was heading to a commemoration in Russia's Katyn forest for thousands of Polish army officers killed by Soviet secret police in 1940 – a massacre the Kremlin denied until 1990.
Prosecutors questioning of Tusk comes amid a high-stakes row between Warsaw and Brussels over rule of law concerns that could see the EU strip Poland of voting rights.
The European Union announced Saturday it had launched legal action against Poland's PiS government over a new court reforms that it fears will erode judicial independence.
Tusk said Thursday that the PiS law "confirms that Jaroslaw Kaczynski is dreaming of a justice system that will answer to him."