South Korea's ousted premier said on Thursday that he had opposed suspended President Yoon Suk-yeol's declaration of martial law, testifying for the first time at his former boss' impeachment trial about the events of a night that threw the country into turmoil.

Han Duck-soo was impeached by parliament as acting President and Prime Minister in December over alleged obstruction of the trial that would formally remove Yoon from office for his martial law decree.

On Thursday, he appeared before that trial for the first time, telling Seoul's Constitutional Court he had "expressed my opposition" to Yoon's decision to suspend civilian rule on the night of December 3.

Han said he and most of his fellow cabinet members "believed such a declaration would put South Korea in serious difficulty", and that he recalled them "being concerned and trying to dissuade it".

Thursday's hearing was Yoon's 10th before Seoul's Constitutional Court.

Once the hearings wrap up, the judges will go behind closed doors to deliberate Yoon's fate — with elections required in 60 days if he is removed.

Yoon walked out of the court just five minutes after proceedings began on Thursday, according to a pool report.

His lawyer Yoon Kap-keun told reporters that the ousted president felt it was "inappropriate" for him to sit in the same court room with Han "or for the president to watch the prime minister testify".

"It is not good for the nation's prestige," his lawyer quoted Yoon as saying.

Yoon later returned to hear the testimony of former senior intelligence official Hong Jang-won, seen as a key figure in the decision to declare martial law.

Hong has claimed to be in possession of a memo containing a list of names of individuals Yoon ordered arrested during the night of the martial law declaration, including the leaders of the opposition and Yoon's own ruling party.

"I will do my best to recount everything as I remember it," Hong told reporters before the hearing.

The head of South Korea's National Police Agency, who is also on trial on insurrection charges related to the martial law decree, is another witness. (AFP)