South Korean police have begun investigating President Yoon Suk-yeol for alleged "insurrection" over his declaration of martial law, a senior police officer said on Thursday.

Woo Jong-soo, head of the National Investigation Headquarters of the National Police Agency, told lawmakers that "the case has been assigned," footage showed

President Yoon's office said there would be no public statements on Thursday.

"President Yoon will not make any statements today," a presidential official told reporters, as the opposition set an impeachment vote for 7pm on Saturday.

Earlier, Yoon accepted the resignation of his defense minister as opposition parties moved to impeach both men over the stunning yet short-lived imposition of martial law that brought armed troops into Seoul streets.

The main opposition Democratic Party and other small opposition parties submitted a joint motion to impeach President Yoon on Wednesday over his martial law declaration the previous night.

Martial law lasted about six hours, as the National Assembly quickly voted to overrule the president, forcing his Cabinet to lift it before daybreak Wednesday.

On Thursday, Yoon’s office said he replaced Defence Minister Kim Yong-hyun with Choi Byung-hyuk, a retired four-star general who is South Korea’s ambassador to Saudi Arabia.

Vice Defence Minister Kim Seon-ho will be the acting minister until Choi assumes the job after a parliamentary hearing.

The hearing is seen as a formality as the president holds the power to appoint ministers outside of the prime minister without the approval of lawmakers.

Yoon hasn’t made any public appearances since he announced in a televised address that his government was lifting the martial law declaration.

The opposition parties earlier submitted a separate motion to impeach Kim, alleging he recommended Yoon impose martial law.

On Wednesday, Kim offered to resign and apologised for causing disruption and concern to the public.

Kim said “all troops who performed duties related to martial law were acting on my instructions, and all responsibility lies with me,” according to the Defence Ministry.

The impeachment motion on Yoon was introduced at a parliamentary plenary session early Thursday, meaning it can be put to a vote between Friday and Sunday.

By law, the motion will be scrapped if it isn’t voted within 72 hours of its parliamentary introduction.

Opposition parties can submit a new impeachment motion if the current one is scrapped or voted down, according to National Assembly officials. (Agencies)