Yoon is was removed from office in April by the Constitutional Court over his botched attempt in 2024 to declare martial law and is being investigated by a special prosecution team formed under new President Lee Jae-myung.
Prosecutors investigating influence-peddling allegations connected to Yoon and his wife had tried to get him to comply with an arrest warrant and attend questioning voluntarily, a spokesperson for the special prosecution said.
"But the suspect stubbornly refused to do so, while lying on the ground, not dressed in a prison uniform," Oh Jung-hee told a news briefing on Friday.
She said investigators would try again to bring him in, even if they had to use force.
Yoon was dressed only in his undershirt and underwear when prosecutors went to his cell, the Yonhap News Agency reported, citing the special prosecution.
Yu Jeong-hwa, one of Yoon's lawyers, told Reuters that bringing up what he was wearing in a small space where the temperature was close to 40C was a public insult to his dignity and showed how the state was violating inmates' human rights.
The former president was put back in a solitary cell at the Seoul Detention Centre in July as prosecutors investigating his short-lived declaration of martial law in December sought additional charges against him.
Yoon is already on trial for insurrection, a charge which is punishable by death or life imprisonment.
He also faces a string of other investigations led by special prosecutors including one into scandals surrounding his wife, former first lady Kim Keon-hee, where the couple allegedly exerted inappropriate influence over elections.
Yoon has denied any wrongdoing and his lawyers have accused prosecutors of conducting a politically-motivated witch hunt.
The former president has repeatedly rejected requests by prosecutors to appear for questioning, citing health issues, including a condition that means there is a risk of him losing his eyesight.
In a reference to Yoon's position as a former top prosecutor, Oh, the spokesperson for the special prosecution, said the case was being closely scrutinised by the public, which was watching to see if the law applied equally to everyone.
Separately, investigators requested that Yoon's wife Kim, who has also denied any wrongdoing, attend questioning on August 6.
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