SEOUL - Hundreds of thousands of South Koreans are expected to rally during the upcoming weekend to demand President Park Geun-hye's resignation over scandal involving her longtime confidante and former aides.

A private association aiming to let President Park step down, which is composed of about 1,500 civic groups, announced a plan to hold a massive rally in a city hall square in central Seoul this Saturday.

It would be the third weekend protest since the scandal over the president's decades-long friend, Choi Soon-sil, came into focus last month. Choi, who has been placed under custody, is charged with meddling in state affairs behind the scenes and peddling undue influence for personal gains.

Police estimates 160,000-170,000 protesters would attend the rally. Organizers said at least 500,000 would turn out, according to Yonhap news agency report on Thursday. Last Saturday, about 200,000 marched in Seoul to demand the embattled president's resignation.

This Saturday, the Korean Confederation of Trade Unions, one of the country's two umbrella labor union groups, expected to bring at least 100,000 unionists and supporters to the square.

Voluntary participants, including couples, families and students as seen in last Saturday's protest, are expected to peak in number. Three main opposition party members vowed to join the rally.