The Constitutional Court is expected to decide in the coming days whether to oust Yoon in a case that ignited the nation’s worst political crisis in decades and rattled markets.
In central Seoul, anti-Yoon protesters filled a large square, chanting for his immediate removal, and were joined by opposition politicians.
A few blocks away, conservative Yoon supporters crammed an entire avenue, calling for his return and waving South Korean and American flags.
The major opposition Democratic Party said a million people attended the anti-Yoon rally, while police put the number at each demonstration at 43,000, Yonhap news agency reported.
Yoon is also on trial on a criminal charge of insurrection, although he was freed from detention last week.
His martial law imposition and its fallout have widened deep social rifts between conservatives and liberals and put pressure on institutions and the military, which found itself in a quandary over whether to enforce martial law.
Pro- and anti-Yoon demonstrators have been taking to the streets in their hundreds of thousands, week after week, since the crisis started.
"Last week, I thought that the Constitutional Court would rule, but it didn’t. Then Yoon was released, making me incredibly frustrated," said 48-year-old protester Song Young-sun. "So this week I came here, hoping that the Constitutional Court will rule on the impeachment case next week."
In a Gallup Korea poll published on March 14, 58% supported Yoon’s impeachment, while 37% opposed it.
"I hope that the judges of the Constitutional Court will make a precise judgment and dismiss the case," said Kim Hyung-joon, a 70-year-old pro-Yoon protester.