Romania's former president was booed and jeered as he went to face prosecutors who have put him under criminal investigation for allegedly making threatening remarks to a senator.

As he entered the prosecutor's office Tuesday, former President Traian Basescu smiled and said it was "normal" he was being investigated. He potentially faces charges of threatening and blackmailing Senator Gabriela Vranceanu-Firea, a political rival.

Some 20 protesters gathered outside yelling: "Basescu, to prison!" He was president from 2004 to 2014.

Last year, while Basescu was still president, Vranceanu-Firea accused Basescu's elder daughter of fraud related to a property restitution case.

Basescu then publicly warned Vranceanu-Firea to "be careful," saying one day she might not find her husband at home. The investigation against him began on March 11.