SEOUL — South Korean President Yoon Suk Yeol is facing mounting pressure to resign over his failed attempt to impose martial law, as parliament moves closer to a vote on his impeachment.

The capital Seoul has seen a second day of street protests demanding Yoon's resignation while police said he was being investigated for "insurrection".

The opposition is pushing for a vote on the motion to impeach Yoon on Saturday, even as the president's party has said that they are against it.

It needs at least eight members of Yoon's party to vote for his impeachment for the motion to pass with a two-thirds majority in the 300-seat parliament.

South Korea was plunged into political turmoil late on Tuesday night when Yoon made the shock martial law declaration.

He cited threats from "anti-state forces" and North Korea. However, it soon became clear that his move had been spurred not by external threats but by his own domestic political troubles.

Yoon rolled back the declaration six hours later after MPs voted it down. Some lawmakers jumped over barricades and fences to get past security forces in order to convene in parliament and void Yoon's decree.

His attempt to rule by martial law has caused public outrage and street protests. He has not spoken publicly since revoking his decision early on Wednesday.

Before his attempt to place the country under military rule, Yoon had been beset by low popularity ratings, corruption allegations and an opposition-led legislature that reduced him to a lame duck leader.

More anti-Yoon rallies are due to take place in Seoul later on Thursday. They are expected to grow larger as people join in after work or school. There have also been some rallies in support of him but they have been much smaller.

Some lawmakers are also staying near the National Assembly to make sure that they are ready to void another martial law decree, should Yoon attempt it again.

Authorities have started investigating Yoon for alleged insurrection following a complaint by the opposition, said Woo Jong-soo, head of the National Investigation Headquarters of the National Police Agency. Insurrection is punishable by death and transcends presidential immunity.

The National Assembly also started investigating the martial law declaration on Thursday.

"The Yoon Suk Yeol regime's declaration of emergency martial law caused great confusion and fear among our people," opposition Democratic Party lawmaker Kim Seung-won said.

Yoon's defence minister, Kim Yong-hyun, resigned on Thursday and took full responsibility for the martial law declaration.

It was Kim who suggested martial law to Yoon, Interior Minister Lee Sang-min told the parliament hearing. South Korean media have carried similar reports.

It became clear during the hearing that few knew about Yoon's plan. The vice defence minister, Kim Seon-ho, said he learnt about it on the news.

Army chief Park An-su said he had been handed the text of the martial law notice only on Tuesday night.

"There were four of us who were reviewing the draft... While we are military experts, we are not experts on martial law. Time was passing and we were all saying ‘what are we to do, what are we to do?' he told MPs.

A motion to impeach Yoon was tabled in parliament late on Wednesday, and South Korean law requires that it be put to a vote within 72 hours.

The main opposition Democratic Party is pushing for a vote on Saturday evening.

If the impeachment is approved, the president will be immediately suspended from office, while the prime minister becomes acting president.

A trial will then be held before the Constitutional Court, a nine-member council that oversees South Korea's branches of government. The process could take up to 180 days.

If six of the court's members vote to sustain the impeachment, the president will be removed from office. — BBC