The feud between the European Council president and the ruling party in his native Poland has intensified as he arrived in the country denouncing a "witch-hunt" against him.
Donald Tusk has been summoned to give evidence to an investigation.
Throngs of supporters escorted him from Warsaw's central railway station to the prosecutor's office.
Mr Tusk was Polish prime minister from 2007-14. His Civic Platform party is now the main opposition.
Its chief rival at the time was the nationalist Law and Justice Party (PiS), which is now in power.
PiS leader Jaroslaw Kaczynski holds Mr Tusk politically responsible for the 2010 plane crash in Smolensk, Russia, which killed his twin Lech Kaczynski, as well as 95 others on board.
The government has also accused Mr Tusk of abusing his EU position to meddle in domestic affairs.
Last month, Poland was the only one of 28 EU member states to vote against Mr Tusk's re-election as president of the European Council. To the visible fury of Polish Prime Minister Beata Szydlo, he was granted another two-and-a-half-year term.
Now Mr Tusk is being questioned as part of an investigation into whether two former leaders of the country's military counter-intelligence service (SKW) exceeded their authority in agreeing to co-operate with a foreign intelligence service in 2010 and 2013.
According to reports, at stake is whether Mr Tusk - then PM - authorised this alleged agreement with Russia's Federal Security Service (FSB).
"I have no doubt that this is part of a political witch-hunt," Mr Tusk said. Its outcome rested in the "hands of the people", he added according to AFP news agency.
Correction 4 May 2017: A description of the investigation as "secretive" has been removed from this report.