Tens of thousands of protesters have gathered in South Korea’s Seoul for the fourth in a weekly series of demonstrations aimed at forcing President Park Geun-Hye to resign over a corruption scandal.

Saturday’s protest came as Park’s approval ratings plunged with prosecutors planning to interview her, making her the first sitting South Korean president to be questioned in a criminal case.

The scandal centres on Park’s shadowy confidante Choi Soon-sil, who is accused of using her ties with the president to coerce local firms to donate millions of dollars to non-profit foundations that Choi then used for personal gain.

After claiming a turnout of about one million for last week’s protest, organisers said they expected some 500,000 people on Saturday, while police predicted one-tenth that number.

So far the protests have been largely peaceful, with many families participating, but there was still a heavy police presence, with buses and trucks blocking access roads to the presidential Blue House.

“There have been protesters on the streets in smaller numbers throughout Saturday. But the numbers have built up significantly as we approached the official start of the demonstration [in the evening],” Al Jazeera’s Wayne Haye said, reporting from Seoul.

“It seems that hundreds of thousands of people are going to be here by the time this protest comes to an end later.”

The president has defied calls to step down, but her lawyer said that she would cooperate with public prosecutors who sought to question her next week.

Prosecutors said that they planned to formally charge Choi by Sunday. They are also seeking to indict two of Park’s former aides who allegedly conspired with Choi.

Separately, opposition parties used their parliamentary majority to pass a law that would allow for a special prosecutor to investigate the scandal.

READ MORE: Park Geun-hye: Scandal is all my fault and mistake

Opposition parties have yet to seriously push for Park’s impeachment because they fear triggering a backlash from conservative voters, which could hurt them in next year’s presidential election.

However, there is a growing voice within the opposition that says an impeachment attempt is inevitable because it is unlikely Park will resign and give up her immunity from prosecution.

Park’s term lasts until February 24, 2018. If she steps down before the presidential vote on December 20, 2017, an election must be held within 60 days.