SEOUL — Tens of thousands of South Koreans poured into the streets of downtown Seoul on Saturday, using words including ‘‘treason’’ and ‘‘criminal’’ to demand that President Park Geun-hye step down amid an explosive political scandal.
The protest, the largest antigovernment demonstration in the capital in nearly a year, came a day after Park apologized on live television amid rising suspicion that she allowed a mysterious confidante to manipulate power from the shadows.
Holding banners, candles, and colorful signs that read ‘‘Park Geun-hye out’’ and ‘‘Treason by a secret government,’’ a sea of demonstrators filled a large square in front of an old palace gate and the nearby streets, singing and thunderously applauding speeches calling for the ouster of the increasingly unpopular president.
They then shifted into a slow march in streets around City Hall, shouting ‘‘Arrest Park Geun-hye,’’ “Step down, criminal,’’ and ‘‘We can’t take this any longer,’’ before moving back to the square and cheering on more speeches that continued into the night.
‘‘Park should squarely face the prosecution’s investigation and step down herself. If she doesn’t, politicians should move to impeach her,’’ said Kim Seo-yeon, one of the many college students who participated in the protest.
‘‘She absolutely lost all authority as president over the past few weeks,’’ he said.
Earlier in the week, prosecutors arrested Choi Soon-sil, the daughter of a late cult leader and a longtime friend of Park, and detained two former presidential aides over allegations that they pressured businesses into giving $70 million to two foundations Choi controlled.
There are also allegations that Choi, despite having no government job, regularly received classified information and meddled in various state affairs, including the appointment of ministers and policy decisions.
‘‘I came out today because this is not the country I want to pass on to my children,’’ said another demonstrator, Choi Kyung-ha, a mother of three. ‘‘My kids have asked me who Choi Soon-sil was and whether she’s the real president, and I couldn’t provide an answer.’’
Choi Tae-poong, a 57-year-old retiree, said he came out to protest because he thought the situation had reached a point where ‘‘no more patience is allowed.’’
‘‘I cannot bear this anymore,’’ he said.
Police estimated the crowd at 45,000, although protest organizers said about 200,000 people turned out.
Police used dozens of buses and trucks to create tight perimeters in streets around the square in front of the palace gate to close off paths to the presidential office and residence. Thousands of officers dressed in fluorescent yellow jackets and full riot gear stood in front of and between the vehicles as they closely monitored the protesters.
Smaller protests have taken place in the past few weeks in Seoul and other cities amid growing calls for Park to step down. While several politicians have individually called for Park’s ouster, opposition parties have yet to attempt a serious push for her resignation or impeachment in fear of negatively impacting next year’s presidential election.
‘‘How many more astonishing things must happen before this country changes for the better?’’ said Park Won-soon, the opposition mayor of Seoul and a potential presidential candidate, vowing to push for the president’s resignation.
President Park has tried to stabilize the situation by firing eight aides and nominating three new top Cabinet officials, including the prime minister, but opposition parties have described her personnel reshuffles as a diversionary tactic.
One national poll released Friday had Park’s approval rating at 5 percent, the lowest for any president in South Korea since the country achieved democracy in the late 1980s following decades of military dictatorship.
In Friday’s televised apology, Park commented on the corruption allegations surrounding Choi and her former aides and vowed to accept a direct investigation into her actions, but avoided the more damning allegation that Choi perhaps had interfered with important government decisions on policy and personnel.
Opposition parties, sensing weakness, immediately threatened to push for her ouster if she doesn’t distance herself from domestic affairs and transfer the duties to a prime minister chosen by parliament. The parties have also called for a separate investigation into the scandal led by a special prosecutor.
Park has 15 months left in her term. If she resigns before the end of it, South Korean laws require the country to hold an election to pick a new president within 60 days.