Supporters of President Yoon Suk Yeol, left, stage a rally near Gwangwhamun Squre in Seoul, while anti-Yoon supporters gather near the Constitutional Court in Seoul, March 8. Yonhap

Tens of thousands of people were expected to gather in Seoul on Sunday to rally for or against impeached President Yoon Suk Yeol a day after his surprise release from detention.

Sarang Jeil Church, led by conservative activist pastor Jeon Kwang-hoon, held a Sunday service outdoors near the presidential residence in central Seoul. Around 4,500 people had gathered as of noon, according to an unofficial police estimate.

"With President Yoon's release, the impeachment trial has become meaningless. It's over," Jeon said. "In the event the Constitutional Court does something funny, we will exercise the people's right to resist and get rid of them with a single slash."

On Saturday, Yoon was released from the detention center where he had been held since mid-January over his failed martial law bid in December, after a court ruled that his detention was invalid.

Even in the absence of physical detention, Yoon still has to stand trial on charges of leading an insurrection and is awaiting the verdict of the Constitutional Court on whether to uphold or dismiss his impeachment.

Conservative civic group Angry Blue was scheduled to hold a separate rally, opposing Yoon's impeachment at Bosingak Pavilion at 1 p.m. before marching along Jongno 3-ga street.

An opposing rally by a group of activists calling for Yoon's immediate ouster was held overnight just outside of Seoul's Gyeongbok Palace, and the group held a press conference outside the nearby government complex Sunday to declare a week of "emergency action" urging Yoon's removal from office.

At 2 p.m., the group will begin a march from the National Palace Museum, with the reported attendance of 100,000 people, leading to partial road closures in the area.

At 7 p.m. it will hold another rally in Gwanghwamun.

Meanwhile, the main opposition Democratic Party of Korea demanded Prosecutor General Shim Woo-jung to resign immediately to take responsibility for the prosecution's release of Yoon following the court ruling.

The party threatened to consider all possible measures, including his impeachment, should the top prosecutor refuse.

"The Democratic Party of Korea will immediately file a complaint against Prosecutor General Shim Woo-jung," floor leader Park Chan-dae said after an emergency general meeting of lawmakers. "Prosecutor General Shim released Yoon Suk Yeol, the ringleader of an insurrection, after voluntarily giving up the opportunity to immediately appeal the court decision (to release him) and receive the judgment of a higher court."

The ruling People Power Party stepped up its calls on the Constitutional Court to dismiss Yoon's impeachment and reinstate him.

"The Constitutional Court must keep in mind that it will face unbearable backlash in the event it hastily removes the president from office and he is later acquitted of the insurrection charges," a party spokesperson said. (Yonhap)