Pressure on South Korea's scandal-hit president to resign escalated sharply Saturday, with organisers claiming a million-strong turnout at one of the largest -- and loudest -- anti-government protests the country has ever witnessed.

Chanting "Step down Park Geun-Hye!" enormous crowds, including high school students, Catholic nuns, labourers, farmers, retirees and young couples with babies, massed in the streets of central Seoul in a powerful display of popular anger and dissent.

While police put the turnout at 260,000, organisers said one million people took part in what was the third in a series of weekly protests over a corruption scandal that has left Park fighting for her political survival.

On the back of official appeals for calm, police deployed around 25,000 officers, many of them in full riot gear, while police buses and trucks blocked every access road -- major or minor -- around the presidential Blue House.

As night fell, Seoul's main ceremonial boulevard Gwanghwamun became a moving river of flickering candles held by the banner-waving, slogan-chanting demonstrators calling on Park to quit.

"It was our wedding anniversary yesterday but we cancelled our anniversary trip and came to Seoul because we thought it was more important for our daughter," said Cho Joo-Pyo, who came with his wife and their two-year-old.

Cho's family had travelled from Jeonju, around 200 kilometres (120 miles) south of Seoul -- one of tens of thousands who travelled from cities across the country to participate in the biggest anti-government rally since the pro-democracy protests of the late 1980s.

- Fraud and scandal -

The scandal engulfing Park for the past three weeks has focused on her close friend, Choi Soon-Sil, who is currently under arrest on charges of fraud and abuse of power.

Prosecutors are investigating allegations that Choi, 60, leveraged their friendship to coerce donations from large companies like Samsung to non-profit foundations which she set up and used for personal gain.

She is also accused of interfering in government affairs, despite holding no official position.

Lurid reports of the unhealthy influence Choi wielded over Park have seen the president's approval ratings plunge to five percent -- a record low for a serving president.

And Saturday's rally was a focal point for a litany of other complaints, from plunging rice prices to the government's handling of the 2014 Sewol ferry disaster.

One group dressed in traditional funeral gear carried a large "presidential coffin" covered with a banner reading: "Step down Park Geun-Hye, killer of agriculture, farms and farmers."

The family-friendly protest was peaceful but sometimes reached deafening volumes as performers and activists whipped the crowds into a raucous frenzy from a giant stage.

"We're so close to the Blue House. Let her hear us roar!" yelled one speaker who was rewarded with a massed bellow of approval.

- Spurned apologies -

In an effort to soothe public anger, Park has issued several public apologies, voicing her personal "heartbreak" at being the cause of such widespread anger and distress.

She has also reshuffled top officials and even agreed to relinquish some of her extensive executive powers, but the popular calls for her to step down have been relentless.

"She has apologised but I don't think it was sincere," said college student Ahn Ye-jin.

"As Korean citizens, it is up to us to bring about change in this country and that is why I am here today. Park has to go," Ahn said.

Most experts have suggested the president, who has just over a year left of her single five-year term, will be able to ride out the crisis and remain in office, albeit with her authority and ability to govern seriously undermined.

Opposition lawmakers have largely avoided direct resignation calls and appear more interested in extracting further concessions from Park in terms of devolving power to the legislature.

But the sheer size and volume of Saturday's demonstration will be impossible to just ignore.

"Judging from what I saw today, people's resentment against her is simply too strong for Park to stare this down," said Lee Yeon-Ho, a political science professor at Yonsei University.

"People are literally seething with anger," Lee said.