Hundreds of emotional people gathered on a train platform in Warsaw to await the arrival of Donald Tusk, president of the European Council and former Polish prime minister, who is to testify in an investigation into an alleged secret deal between Polish and Russian intelligence officials.

The mood was tense as they awaited Tusk's arrival on Wednesday, with supporters holding EU flags and his detractors holding up signs accusing him of crimes.

Tusk is to be questioned by prosecutors as a witness. Many see his questioning as part of a larger attempt by Poland's nationalist government to discredit him and perhaps imprison him eventually. He has been accused by the defense minister of treason in another matter, the handling of the 2010 plane crash that killed the Polish president.