About 1 million South Koreans are expected to join a mass rally nationwide today to demand scandal-hit President Park Geun-hye steps down, China's Xinhua news agency reported organisers as saying.
In Seoul, around half-a-million demonstrators are forecast to march at night with paper cup-wrapped candles in hand.
Last Saturday, over 1.3 million people took to the streets in the capital city.
Another half-a-million would turn out in some 50 major cities across the country in the fourth Saturday demonstration since the scandal involving President Park and her longtime confidante burst into a rage last month.
Protesters dotted across the capital city marched in early afternoon to the Gwanghwamun Square in central Seoul, just over a kilometre away from the presidential Blue House, where presidential office and residence are located.
An official event around the square is scheduled to kick off at 6pm local time, two hours later than the previous week.
Before the official event, three main opposition parties held separate rallies around the square. Presidential hopefuls in the opposition bloc are scheduled to join candlelight vigils in major cities outside Seoul.
As seen in last Saturday's rally, people attending the protest varied from young couples with children and the grey-haired elderly to secondary school students and labor unionists.
People were enraged further as Park delayed a face-to-face questioning by prosecutors to next week. Park said in her second public apology that she would accept an investigation on herself if necessary.
The prosecution office had asked Park to be investigated no later than Wednesday, requesting it once again as late as Friday. However, it did not happen.
Choi Soon-sil, Park's longtime confidante suspected of intervening in state affairs though she has no public position, and two former presidential secretaries are set to be indicted by prosecutors tomorrow.
Prosecutors said the embattled president's status could be changed into a criminal suspect as Park's "criminal charges" could be in question in the indictment.