Organisers said the mass rally would march on the Constitutional Court whose nine justices are considering the validity of the impeachment bill.
Organisers said the mass rally would march on the Constitutional Court whose nine justices are considering the validity of the impeachment bill passed by the national assembly more than a week ago.
The court has 180 days to make a ruling, but the protesters are pressing for a swift judgement.
Although Park has been stripped of her substantial executive powers, she is allowed to retain the title of president and continue to live in the presidential Blue House while the court deliberates.
The protesters are adamant that she should resign immediately and face criminal prosecution.
But Park still has her supporters, many of them elderly voters who remain steadfast admirers of her father, the late military dictator Park Chung-Hee -- credited as the architect of the South's economic transformation but vilified as an authoritarian rights abuser.
Several thousand Park loyalists attended their own rally near the court earlier in the day to demand the impeachment bill be thrown out.
Waving national flags and clutching red roses they carried banners denouncing the anti-Park protests as a leftist conspiracy.
On Friday, Park's legal team formally submitted a 24-page rebuttal of the impeachment charges to the court, arguing that they had no legal basis.
"We can't accept that there was any violation of the constitution by the president... the impeachment motion should be rejected," one of her lawyers, Lee Joong-Hwan, told reporters.
Park was impeached on numerous counts of constitutional and criminal violations ranging from a failure to protect people's lives to bribery and abuse of power.
Most of the charges stemmed from an investigation into a scandal involving the president's long-time friend, Choi Soon-Sil, who is currently awaiting trial for fraud and embezzlement.
Prosecutors named Park a suspect in the case -- a first for a sitting president -- saying she colluded in Choi's efforts to strong-arm donations from large companies worth tens of millions of dollars.
The impeachment process was ignited and fuelled by public outrage at Park's behaviour, with the weekly mass demonstrations demanding that politicians take a proactive role in removing her from the presidential Blue House.
The National Assembly has played its part, but the country now faces a lengthy period of uncertainty at a time of slowing economic growth and elevated military tensions with nuclear-armed North Korea.
The man charged with steering the country through these dangerous waters is a former prosecutor who has never held elected office.
As Park's prime minister, Hwang Kyo-Ahn became the temporary guardian of her sweeping executive powers the moment after she was impeached.
The protesters have called on him to resign as well, arguing that he is too tainted by his association with the president to wield her authority.
The mass rallies, some of which drew crowds of more than one million people, have been passionate but good-natured so far, with no clashes despite a heavy police presence.