Hosting what she called a "thanksgiving" lunch for scores of journalists at her office instead of meeting with government investigators, she accused the government of plotting to have her removed from office and charged in court.
Duterte, 46, had been subpoenaed following a November 23 press conference where she claimed to have told someone to kill Marcos should an alleged threat against her own life be carried out -- comments she later said were misinterpreted.
The alliance between Duterte and Marcos -- her partner in a landslide 2022 presidential election victory -- has collapsed spectacularly in the lead-up to next year's midterm polls.
"I do not believe that the investigation will be impartial," Duterte told reporters, explaining her absence at the inquiry.
"The worst-case scenario that we're seeing is (my) removal from office, impeachment, and then layer upon layer of cases."
Two political parties formally asked the House of Representatives two weeks ago to remove her from office through impeachment.
Marcos however said he has urged the House not to impeach her, saying he considered such an attempt pointless.
"If her reason for not showing up is a Christmas party, that is not justifiable," National Bureau of Investigation director Jaime Santiago told reporters Wednesday when asked about Duterte's decision.
"If you are accused of something and you don't respond, your silence condemns you."
Duterte last month delivered an expletive-laden online news conference in which she claimed to have told someone to kill Marcos, his wife Liza, and House of Representatives Speaker Martin Romualdez -- a Marcos cousin -- if she were assassinated.
"If I die, don't stop until you have killed them," she said, adding that she was "not joking".
She later denied making a death threat, describing her comments as an expression of "consternation" with the Marcos administration's failures.
Both Romualdez and Duterte are widely expected to run for president in 2028.
