Waving banners and signs demanding the ouster of the conservative president, big crowds of anti-Yoon protesters packed the streets near the court, where police had recently tightened security in anticipation of the ruling expected as early as next week.
Yoon's supporters rallied in nearby streets, waving South Korean and US flags while calling for the return of their conservative hero, whose ill-conceived power grab evoked memories of the military dictatorships last seen in the 1980s.
Police deployed thousands of officers to maintain safety and there were no immediate reports of major clashes or injuries.
Organisers of the anti-Yoon protests estimated the turnout at 1.1 million, while police put the size in the tens of thousands.
Marching toward streets near the court, the protesters sang and chanted slogans demanding Yoon's ouster and imprisonment over his short-lived martial law imposition on December 3.
Yoon's powers were suspended after the opposition-controlled National Assembly impeached him on December 14.
"We cannot wait even a single day," one of the protest leaders said on stage.
"This is the order of our citizens — the Constitutional Court must immediately remove Yoon Suk Yeol, the ringleader of rebellion!"
To formally remove Yoon from office, at least six of the Constitutional Court's eight incumbent justices must approve the impeachment motion passed by MPs.
If they do not, Yoon's presidential powers will be immediately restored. Chung Sung-il, a 72-year-old anti-Yoon protester, said he expected the court to dismiss Yoon, "100 per cent".
"If he is reinstated, so many dangerous things can happen."
The pro-Yoon rallies were attended by members of Yoon's conservative People Power Party, including five-term lawmaker Yoon Sang-hyun.
He has defended the president's martial law imposition as an attempt to check against the "legislative dictatorship" of the liberals, who have obstructed his agenda with their majority in the assembly.
The crowds repeatedly chanted Yoon Suk Yeol's name and held signs that read: "Dissolve the National Assembly".
"President Yoon declared martial law to protect free democratic South Korea," said Jin Woo-chan, a 20-year-old Yoon supporter.
Yoon has argued that his martial law decree was necessary to overcome the "anti-state" liberal opposition, which he claims improperly used its legislative majority to block his agenda.
Despite blockades by hundreds of heavily armed troops, MPs gathered a quorum and unanimously voted to lift martial law, hours after Yoon declared it.
The constitution limits the exercise of such powers to times of war or comparable national emergencies.
Yoon's legal saga, which also includes a separate criminal indictment on rebellion charges, rattled state affairs, diplomacy and the economy and has become a stress test for the country's democracy.
Yoon's conservative supporters rioted at a Seoul court that authorised his arrest.
His lawyers and the ruling party have openly questioned the credibility of courts and law enforcement institutions, and Yoon has continued to express contempt for his liberal rivals, endorsing baseless conspiracy theories about election fraud to justify his ill-fated authoritarian push.
If the Constitutional Court dismisses Yoon, that will trigger a presidential by-election within two months.
