Yoon faces a fateful impeachment vote after his short-lived martial law declaration earlier this week, which sparked nationwide outrage.
Police estimated that around 149,000 people had gathered as of 5:30 p.m., as many citizens and civic groups packed the streets.
The union criticized Yoon's overnight martial law declaration on Tuesday, calling it an "act of treason" and demanding the dissolution of the ruling People Power Party (PPP), which opposes his impeachment.
Protesters also called for the immediate arrest and impeachment of the president.
Meanwhile, subway trains are bypassing Yeouido Station, and the area near National Assembly Station including the southern section of Seogang Bridge is fully blocked.
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