Portnov wrote on Facebook that his questioning on July 29 was an attempt to "distract me from my work on [former Ukrainian Presidnet Petro] Poroshenko."

"I think their main goal was...to give me a signal to make me slow down my activities against Poroshenko. We will not slow down...There is no point in trying to frighten us," Portnov told Hromadske TV.

Portnov, who fled Ukraine after Yanukovych was toppled in late February, 2014, returned to the country on May 19.

After coming back a day before President Volodymyr Zelenskiy's inauguration, he filed several lawsuits against Poroshenko accusing him of crimes including economic misdeeds and illegal attempts to retain power.

With reporting by Hromadske TV