The demonstration is due to begin at 11:00 local time at Plac Defilad in central Warsaw. An hour earlier, President Karol Nawrocki is scheduled to meet farmers involved in organising the protest.
Farmers say the deal could flood Polish and wider European markets with significantly cheaper food imports, threatening local producers.
The timing of the protest coincides with a vote taking place in Brussels on Friday, where EU member state ambassadors are due to decide on safeguard clauses in the Mercosur agreement.
The proposed deal would grant preferential access to EU markets for selected agricultural products from Brazil, Argentina, Paraguay and Uruguay, while opening Mercosur markets to European industrial goods.
European Commission President Ursula von der Leyen had hoped to formally sign the agreement in Brazil in late December, but failed to secure sufficient support from EU member states.
This week, she proposed new financial incentives for farmers, including faster access to €45bn in EU funding from 2028 and earmarking 10% of future national and regional budgets for rural investment.
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Source: IAR/PAP