Yoon accused the main opposition party of sympathising with North Korea and engaging in anti-state activities, according to news reports.
In a televised address, he vowed to eradicate what he described as "shameless pro-North Korean anti-state forces" and said that declaring martial law was necessary to protect constitutional order, the Reuters news agency reported.
The opposition Democratic Party said in response that Yoon's move was unconstitutional and illegal, according to US broadcaster CNN.
South Korean lawmakers voted to reject Yoon's decision, and protesters gathered outside the parliament in Seoul, Reuters reported.
Live television footage showed troops attempting to enter the building, it said.
"We are monitoring the political situation in South Korea," Polish Deputy Prime Minister and Defence Minister Władysław Kosiniak-Kamysz wrote in a social media post.
"We are in constant contact with our attaché in Seoul and his South Korean counterpart in Poland," he added.
Władysław Kosiniak-Kamysz. Photo: Przemysław Chmielewski/Polish Radio
Kosiniak-Kamysz also said: "We have received assurances from Deputy Defence Minister Il Sung, on behalf of the South Korean Ministry of Defence, that our cooperation and the implementation of arms contracts are not in any way threatened."
Poland has signed a series of defence deals with South Korea over the past two years to acquire tanks, howitzers and missile launchers in an effort to bolster its military capabilities in response to Russia's invasion of Ukraine.
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Source: IAR, PAP, Reuters, CNN
Click on the audio player above for a report by Radio Poland's Michał Owczarek.