MPs voted to impeach Yoon last Saturday, after his actions sparked widespread protests calling for him to step down.

The court now has six months to decide whether to remove Yoon or reinstate him.

Yoon’s ruling People Power Party (PPP) has been grappling with the fallout ever since, with its leader announcing his resignation on Monday.

While public hearings for Yoon’s impeachment trial could take months, the court is under pressure to decide quickly and bring an end to political uncertainty.

Protesters have vowed to keep up their calls for Yoon’s removal during court proceedings.

In the previous two instances a South Korean president faced impeachment, the court reversed one decision and upheld the other.

Observers say what will complicate Yoon’s trial is the fact that the court only has six justices out of nine, as three had retired recently and have yet to be replaced.

If the court upholds impeachment in Yoon’s case, the country must hold fresh presidential elections within 60 days. In the meantime, Prime Minister Han Duck-soo is serving as acting president.

It is unclear if Yoon will take the stand himself during the court hearings.

He ignored a summons to meet prosecutors in Seoul on Sunday, and is set to be ordered in again for questioning. If he fails to come forward this week, police may move to arrest him.

Yoon has defended his decision to impose martial law, and after his impeachment he once again said he would fight until the end.

Minutes after the constitutional court convened on Monday morning, PPP leader Han Dong-hoon announced his resignation.

“I sincerely apologise to all the people who are suffering from this emergency situation,” Han said in a televised press conference.

Han has faced growing calls for his resignation particularly from the large pro-Yoon faction of his party, following the president’s impeachment.

Han had initially tried to stage an orderly exit for Yoon. But after a defiant speech by Yoon, Han did an about-face and called for his impeachment, saying it was the only way to stop him.

On Saturday, 12 PPP lawmakers voted for impeachment, enabling the motion to pass. But most of the other PPP lawmakers voted against Yoon’s impeachment.

All five Supreme Council members of the PPP said after the vote that they would resign – which would automatically dissolve the party leadership.

As the Supreme Council has been “destroyed”, Han, it was now “impossible” to serve his duties as party chief.

“While it pains me to think of my heartbroken supporters, I don’t regret it,” Han said, in reference to his decision to call for Yoon’s 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.

The late-night pronouncement – made on national TV at 23:00 local time (14:00 GMT) had citizens immediately thinking it was related to North Korea – the South’s nuclear-armed neighbour- or a critical matter of national security like a terrorism event or coup.

However, it became clear fairly quickly that Yoon had made this drastic move in response to a series of political events.

Having lost control of parliament earlier in the year, his government had been battling a series of opposition bills and motions which had sought to undermine his rule.

Political observers say he’s now been driven to the point of invoking martial law – temporary rule by the military – as an undemocratic tactic to fend off the political attacks.

Opposition leaders on Tuesday immediately condemned the move as unconstitutional.

South Korea’s main opposition party leader Lee Jae-myung called on his Democratic Party MPs to converge on parliament on Tuesday night to vote down the declaration.

But in Seoul, police buses had already been moved in to block or barricade the entrance to the parliament building, local broadcasts showed.

Still, demonstrators have rushed to the National Assembly building, protesting and chanting “No martial law! No martial law”. They have clashed with lines of police guarding the building.

Martial law is temporary rule by military authorities in a time of emergency, when civil authorities are deemed unable to function.

The last time it was declared in South Korea was in 1979, when a long-term president had been assassinated during a coup.

It has never been invoked since South Korea became a parliamentary democracy in 1987.

But on Tuesday, Yoon pulled that trigger, in a national address saying he was invoking military rule over a threat from “anti-state forces”.

Under martial law, extra powers are given to the military and there may be a suspension of usual rule of law protections and procedures.

Yoon has been a lame duck president since South Korea’s general election in April when the opposition won a landslide.

His government since then has not been able to pass bills they wanted and have been reduced instead to vetoing bills the opposition have been passing.

He has also seen a dive in his popularity with voters; having been mired in several political influence and corruption scandals – including one involving the First Lady accepting a Dior bag, and another around stock manipulation.

Just last month he was forced to issue an apology on national TV, saying he was setting up an office overseeing the First Lady’s duties.

But he rejected a wider or independent investigation, which opposition parties had been calling for.

Then this week, the opposition proposed slashing budgets for his government – and a budget bill cannot be vetoed.

At the same time, the opposition also moved to impeach cabinet members, including the head of the government’s audit agency – for failing to investigate the First Lady.

Yoon’s declaration caught many off guard – it’s a fast-moving situation now.

The political opposition has called on the public to gather in protest outside parliament – peaceful mass demonstrations are common in South Korea and they have proven effective in changing governments before.

The main opposition Democratic Party on Tuesday night immediately called for all its lawmakers to assemble at the National Assembly.

Under South Korean law, the government must lift martial law if a majority in parliament – the National Assembly – demands it in a vote. The same law also prohibits martial law command from arresting lawmakers.

But police buses had already been moved into place in front of the building in what is being viewed as a barricade to stop lawmakers from reaching the assembly.

There is also discord within Yoon’s own party, the People’s Power Party leader.

Its leader Han Dong-hoon has called the declaration of martial law a “wrong” move, South Korean outlet Yonhap is reporting. He has vowed to block the law.