SEOUL (Reuters) - South Koreans gathered in huge numbers in the capital Seoul on Saturday to support or oppose impeached president Yoon Suk Yeol before a court decides whether his short-lived declaration of martial law disqualifies him from office.

The Constitutional Court is expected to decide in the coming days whether to oust Yoon in a case that ignited South Korea'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 had attended the anti-Yoon rally, while police put the number at each demonstration at 43,000, the 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 had 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.

"Last week, I thought that the Constitutional Court would rule, but it didn't. Then Yoon was released, making me incredibly frustrated," said Song Young-sun, a 48-year-old protester. "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 Friday, 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.

(Reporting by Minwoo Park, Jisoo Kim, Ju-min Park and Daewoung Kim; Editing by Kevin Liffey)