Hundreds of thousands of South Koreans marched in central Seoul on Saturday to surround embattled President Park Geun-hye's office, forming a so-called "human chain" to demand the president's resignation, China's Xinhua news agency reported.

As of 4pm local time, organisers estimated over 200,000 people gathered around the Gwanghwamun Square in central Seoul, just over a kilometre away from the presidential Blue House where Park's office and residence are located.

The demonstrators marched up to 200 metres from the Blue House as a Seoul court allowed it.

Park's office was encircled by a chain of protesters in three sides along the wall of police bus that were parked to prevent an attempt to enter inside the presidential office.

The association to let the Park Geun-hye administration resign, composed of about 1,500 civic groups, held a preliminary event for three hours before the march to the Blue House.

An official candlelight rally is set to kick off at 6pm and the second march would be staged after a lights-out event for a minute from 8pm.

Protesters on the streets are supposed to put out candles, while people watching the nationwide demonstrations on TV are to turn off lights in offices or apartments to indirectly participate in the outside rally.

The advocacy groups' association forecast that around 1.5 million people would take to the streets in the Seoul protest alone.

It would become the country's largest-ever demonstration in downtown Seoul.

At the third Saturday rally on Nov 12, around 1.3 million marched in the capital city, surpassing the June 1987 demonstration that drew about a million protesters.

Today, about 500,000 people are forecast to turn out in major cities across the country, according to the estimate by organisers.

Public anger is escalating as President Park has dismissed growing calls for her voluntary resignation. Instead, the presidential office encouraged politicians to push forward impeachment.

Impeaching President Park could be a political challenge for opposition parties as the impeachment process can take months and cause division.

At least two-thirds of the 300-seat National Assembly is required to vote for the impeachment bill to be passed in the parliament. Opposition parties said it would put the bill to vote as early as next Friday.

By law, the impeachment motion must be endorsed by six out of nine judges of the constitutional court. Two judges are set to end their tenures in January and March next year, indicating the need to get approvals from six of the remaining seven judges.

Today's protest may beat the organisers' expectation given that the previous four rallies ended in an orderly and peaceful manner. Ordinary citizens, ranging from couples with children to secondary school students and grey-haired people, are expected to join the festival-like rally.

Police reportedly mobilised 25,000 police officers in Seoul to brace for a possibility that the rallies turn violent and to prevent some protesters from marching inside the Blue House.

- Bernama