Members of a coalition of civic groups demanding President Yoon Suk Yeol's immediate ousting hold a press conference near Gyeongbok Palace in Seoul, Sunday. Yonhap

Massive rallies by those calling for President Yoon Suk Yeol's immediate ousting and those supporting the Korean president grew more heated and fierce Sunday, just two days after his surprise release from detention.

A coalition of 1,700 progressive civic groups demanding Yoon's immediate removal held an 11 a.m. press conference outside Seoul's Gyeongbok Palace, where they held an overnight rally a day earlier.

The group announced a week of "emergency action," vowing an "all-out war" until Yoon's full removal from office. The campaign began with a massive rally on Sunday afternoon, drawing an estimated 100,000 protesters.

Participants wave light sticks and signs calling for the immediate ouster of President Yoon Suk Yeol during a rally near Gyeongbok Palace in Seoul, Sunday. Yonhap

"The prosecution, which proved to be the subordinate of Yoon Suk Yeol, freed him," the coalition of civic groups said. "The joint effort between the court and the prosecution damaged the constitution and destroyed democracy. Let us gather at the square and dismiss the Yoon Suk Yeol government as soon as possible."

They said they would file a complaint against Prosecutor General Shim Woo-jung with the National Police Agency's National Investigation Headquarters on charges of abuse of authority for granting Yoon's release from detention. From Sunday to the day of Yoon's dismissal, they plan to organize a large-scale rally in central Seoul every day at 7 p.m.

Members of Sarang Jeil Church and conservative pastor Jeon Kwang-hoon hold a Sunday service near the presidential office in Hannam-dong, Seoul. Newsis

Meanwhile, Yoon's supporters gathered in front of the presidential residence in Hannam-dong, calling for Yoon's impeachment to be dismissed.

Led by outspoken conservative pastor Jeon Kwang-hoon, Sarang Jeil Church held a Sunday service near the presidential residence starting at 10:30 a.m., waving Korean and American flags. As of noon, some 4,500 people were at the event, according to a police estimate.

"Now that President Yoon is released, the impeachment trial has become meaningless. It's over," Jeon said. "And if the Constitutional Court attempts to do something wrong? By the name of the right of people to resist, I will blow it away with a strike of a sword."

The pro-Yoon group remained nearby and continued the rally after the service.

With Yoon's return, the Hannam-dong area, which had been the site of a series of protests before his arrest, saw the security and surveillance level strengthened again. The nearby pedestrian overpass was closed and police buses were lined up along Hannam Elementary School, which borders the entrance to the presidential residence.

A conservative civic group Angry Blue also held a separate rally at Bosingak Pavilion at 1 p.m. before marching along Jongno 3-ga street, calling for opposition to impeachment.

The developments come amid a series of large-scale protests that have been held from respective blocs across the country since Yoon's martial law order on Dec. 3. On Saturday, he was released from the Seoul Detention Center where he had been held for 52 days on charges of inciting an insurrection, after a court ruled that his detention was invalid.