SEOUL, Nov. 12 (Xinhua) -- Hundreds of thousands of South Koreans are expected to march in Seoul, the capital city of South Korea, on Saturday night over a scandal involving President Park Geun-hye's longtime confidante and former key aides.

After noon, a number of people began to gather around two squares in central Seoul to attend smaller rallies before the major event that kicked off at 4:00 p.m. local time.

One hour later, attendees are scheduled to march along the road to pre-determined locations near the presidential Blue House.

A private association to demand Park's resignation, which is composed of about 1,500 civic groups, held a pre-official event in the Gwanghwamun Square, just 1 km away from the Blue House where the presidential office and residence are located.

The Korean Confederation of Trade Unions, another organizer and one of the country's two umbrella labor union groups, held an occasion in a Seoul Plaza in front of the city hall, close to the Gwanghwamun square.

The number of demonstrators to attend the Seoul rally is expected to keep rising as labor unionists, youths, university students, scholars, the urban poor, the disabled and farmers plan to join the event after separate protests across downtown Seoul.

Almost all of train tickets for Seoul were sold out, and chartered buses became hard to find as at least 100,000 people left for the capital city to join the mass rally. South Koreans living abroad reportedly plan to hold protest rallies.

Police estimated that 160,000-170,000 people would rally in Seoul, almost quadrupling a crowd of 45,000 turning out a week earlier. The number would be the largest since at least 2008 when around 80,000 protested against then-President Lee Myung-bak over his decision to lift a ban on U.S. beef imports.

Organizers said as much as 1 million demonstrators would take to the streets, far surpassing last Saturday's turnout of some 200,000 in Seoul alone.

Police reportedly deployed some 25,000 officers to prevent any march on the presidential office. Two previous protests on Saturdays were peaceful and calm as ordinary people from couples wheeling a baby carriage to high school students and gray-haired old men marched peacefully.