A South Korean court on Friday ordered the release from jail of Yoon Suk Yeol, the country’s impeached president, who is standing trial on insurrection charges over his decision to impose martial law in December.

The Seoul Central District Court ruled that prosecutors had violated procedural rules by holding Mr. Yoon in detention longer than legally allowed before indicting him last month.

But Mr. Yoon was not immediately freed from the detention center south of Seoul where he is being held, said one of his lawyers, Seok Dong-hyun. Prosecutors have a week to appeal the ruling, during which time Mr. Yoon will remain in custody, Mr. Seok said.

Mr. Yoon was detained on Jan. 15 and formally indicted 11 days later on the insurrection charges, which stem from his short-lived imposition of martial law in December. His lawyers have fought since then to get him freed from jail, making the argument that a three-judge panel accepted on Friday: that prosecutors had held Mr. Yoon longer than the law permitted.

The ruling was limited to a narrow dispute over whether prosecutors had followed all procedures correctly when they arrested and indicted Mr. Yoon. It did not address the charges that Mr. Yoon faces at his criminal trial.

Mr. Yoon unexpectedly declared martial law on Dec. 3, accusing the opposition-controlled National Assembly of “paralyzing” his government. The assembly voted against his martial law decree, forcing him to rescind it after about six hours. But it has set off South Korea’s worst political crisis in decades.

As protesters called for Mr. Yoon’s ouster, the Assembly impeached him on Dec. 14, suspending him from office. The country’s Constitutional Court is deliberating whether the impeachment was legitimate and if he should be formally removed from office. Separately, criminal investigators detained Mr. Yoon on the insurrection charges.

He is the first president in South Korean history to face criminal charges while still in office.