Ahn Jong-Beom was taken into custody late yesterday over suspicions he helped Park's friend Choi Soon-Sil coerce companies into donating large sums to dubious non-profit foundations that she then used for personal gains, Yonhap said.
Ahn, who was dismissed on Sunday, is the second person to be taken into emergency detention after Choi was held Monday for questioning in the influence-peddling scandal.
Park is scrambling to deflect rising public anger over suggestions that Choi - the daughter of a shadowy religious figure - vetted presidential speeches, had access to classified documents, and used her influence for personal enrichment.
"My position is that it is (legally) possible to interrogate and investigate" a sitting president, he said.
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Under South Korea's constitution, the incumbent president may not be charged with a criminal offence except insurrection or treason.
But many argue the sitting president can be probed by prosecutors and then charged after leaving office.
Justice Minister Kim Hyun-Woong also told parliament today that prosecutors could question Park, if the ongoing investigation required it.
The probe also targets firms - including Samsung and SK - that offered donations to foundations favoured by Choi, and a Samsung executive was questioned by prosecutors today, Yonhap added.
- also faces allegations that it separately offered millions of euros to Choi to bankroll her daughter's equestrian training in Germany.
The scandal has shaken the presidency, exposing Park to public outrage and ridicule and, with just over a year left in office, seen her approval ratings plunge into single digits.
In an effort to deflect rising public criticism, Park had been urged to create a neutral cabinet by bringing in members from outside her ruling conservative Saenuri Party.