The EU president has testified as a witness at a probe into the 2010 jet crash that killed Polish President Lech Kaczynski and 95 others. Tusk might himself face charges, if his political enemies have their way.
Former Polish Prime Minister Donald Tusk arrived at the hearing in Warsaw surrounded by supporters chanting his name.
Tusk, who was in office at the time of the crash, said he had no doubt that he was being targeted by his arch-political rival, the Law and Justice (PiS) party leader, Jaroslaw Kaczynski.
Kaczynski's brother, Lech Kaczynski, was president at the time of the fatal crash, and was aboard the plane when it was said to have broken a wing on a tree in poor weather conditions.
Two weeks ago the PiS party leader unleashed a diatribe against Tusk's liberal Civic Platform (PO) party, claiming it was responsible for the crash. "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 lawmakers in parliament.
'An outburst of rage'
Speaking ahead of the hearing on Thursday, Tusk said Kaczynski's words showed his summons to testify had been issued for political reasons.
"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 said.
While Polish and Russian investigators concluded that the disaster could be attributed to human error and bad weather, skeptics have proposed other theories.
Among them is the suspicion that Russia might have placed a bomb on the plane to assassinate the president, who was highly critical of the Kremlin at the time.
Explosion ruled likely
After it won power in 2015, the PiS party launched a new investigation into the tragedy. The commission of inquiry in April said it considered the possibility of an explosion to be quite likely.
Polish prosecutors have also said they will press charges against three Russian air controllers for deliberately causing the 2010 crash.
Among Tusk's fiercest critics is Defense Minister Antoni Macierewicz, who last month accused him of "diplomatic treason" over failures in the post-crash investigation and repatriation of material from the site.
The crash happened as those aboard the plane were on their way to a commemoration of the slaughter of thousands of Polish army officers in Russia's Katyn forest at the hands of the Soviet secret police in 1940.
rc/kms (AP, AFP)