South Korean President Park Geun-hye is facing increasing pressure to resign as hundreds of thousands of demonstrators join nationwide protests over corruption claims.

In what is believed to be one of the largest demonstrations so far over the allegations engulfing Ms Park, organisers said 1.3 million protesters filled the streets in the capital of Seoul on Saturday evening, with 1.5 million estimated to join by the end of the night.

Some 500,000 people are expected to turn out in other regions. Police put the number of participants in Seoul at 260,000.

The demonstrations mark the fifth straight weekend of protests in the country, and are among the largest seen in South Korea since the pro-democracy protests of the 1980s.

Farmers, university students and Buddhist monks were among the demonstrators, who beat drums and chanted "Park get out now", as they marched towards the Presidential Blue House, which had been cordoned over by 24,000 police.

The president is bracing for an impeachment push by opposition parties and some members of her own Saenuri Party. There are allegations she let her long-time confidante Choi Soon-Sil, who has been arrested for fraud and abuse of power, influence government affairs and amass an illicit fortune despite holding no official position - a scandal critics say undermines the country's democracy.

Protesters hold candles during an anti-government rally demanding the resignation of South Korea's President Park Geun-Hye (AFP)

Protesters sit on the street during an anti-government rally demanding the resignation of South Korea's President Park Geun-Hye (AFP)

Sixty-year-old Ms Choi allegedly used her relationship with Ms Park to drive donations from conglomerates, including SK, Lotte and Samsung, to non-profit foundations, which she set up and used for personal gain.

Ms Park has issued two public apologies over the scandal, which have only served to fuel public anger and suspicion, but has resisted repeated calls to resign.

She has promised to submit herself to an expanding probe by prosecutors, as well as a separate investigation by an independent special prosecutor to be appointed by parliament.

The headquarters of SK, Lotte and Samsung were raided by state prosecutors this week, along with the offices of the finance ministry and state pension fund.

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At the rally in Seoul, protesters wearing raincoats and armed with umbrellas against the cold, wet weather, waved placards and lit candles as night descended.

Buddhist monks wearing grey robes recited a sutra while other protestors imitated Ms Park, Ms Choi and Samsung scion Lee Jae-Yong being led into prison.

"I don't think Park would step down voluntarily, but we need to raise our voice as much as possible to encourage parliament to push through with its move to impeach her," Lee Sung-Cheol, a 23-year-old student, told AFP.

"I came here because I wanted to show my children that people are the owner of this country, not the power holders," 47-year-old company employee Shim Ku-Il said.

Kwak Bo-youn, 45, said: "I was watching the news and thought this cannot go on - people really want her to step down but she hasn't. This is the second time for me to the protests, but the first time for my husband and kids".

Police buses are used as barricades to block access roads to the presidential Blue House as protesters take to the streets to press their demand for the resignation of South Korea's President Park Geun-Hye in central Seoul (AFP)

Protesters take to the streets to press their demand for the resignation of South Korea's President Park Geun-Hye in central Seoul (AFP)

Ms Park's approval ratings have plummeted since the allegations emerged. After hovering at just five per cent for three consecutive weeks her approval rating dipped one percentage point on Friday.

According to a poll by Gallup Korea, her disapproval rating has risen three percentage points to 93 per cent. Her lowest approval rating is among young people, with 99 per cent of 19 to 29 year-olds disapproving of the president.

A parliamentary vote to impeach Ms Park, whose five year term ends in February 2018, could take place as early as next week as an increasing number of ruling party politicians back the opposition-led campaign to remove her from office.

If parliament passes the impeachment motion, Ms Park would be suspended from official duties and replaced by the prime minister. The Constitutional Court would need to approve the impeachment.