As the war continued to rage in Ukraine’s east, much of its Western border was blocked on Tuesday by another fight, this one with Polish farmers.

The farmers have for months been protesting an influx of Ukrainian products that they say is crowding the Polish market and undercutting their livelihood. On Tuesday, they obstructed check points for commercial transportation, halted the passage of about 3,000 Ukrainian trucks and opened some train cars containing Ukrainian grain, spilling it onto the rails.

“It’s either us or them,” a Polish farmer said on Tuesday on the Polish TV channel Polsat News. “Someone must be interested in us.”

The demonstration prompted a counterprotest in Ukraine, where previous blockades by Polish truckers have hampered the supply chain of goods reaching the country, causing shortages that have begun to affect soldiers on the battlefield.

Oleh Nikolenko, a spokesman for the Ukrainian foreign ministry, said that the protests “undermine Ukraine’s economy and its resilience to repel Russian aggression.”

The protests on Tuesday echoed those of other farmers across Europe in recent months, which lamented European Union environmental regulations, and imports that the farmers said were making it hard for them to earn a living.

The problems with the Polish farmers originated after Russia’s invasion of Ukraine restricted sea shipments from Ukrainian ports. To ease price hikes and grain shortages in parts of the Middle East, Africa and Asia, the European Union suspended tariffs and quotas on Ukrainian food products in order to carry as much food as possible by rail and truck through neighboring countries such as Hungary, Poland, Romania and Slovakia.

To some degree, the plan worked, but some Ukrainian grain also reached the local markets in the countries it was meant to pass through, creating tensions with local farmers who claimed the influx was pushing down prices.

Before his government was toppled in elections last October, former Prime Minister Mateusz Morawiecki of Poland imposed a unilateral ban on Ukrainian grain and some other farm imports, a violation of European Union rules. The new government led by Donald Tusk has upheld a ban on imports of many products, including sunflower and wheat flour. The agriculture minister said on Tuesday that Poland was working on a bilateral agreement with Ukraine to extend protections to other goods such as sugar, poultry and eggs.

The Ukrainian government said that in January, most of the exports of agricultural products from Ukraine were carried by sea, and only about 5 percent of the total went through Poland.

“This shows that the claims made by Polish agricultural associations about their market being oversaturated are unfair,” Oleksandr Kubrakov, the Ukrainian minister of communities, territories and infrastructure development, said in a statement.

But some signs attached to the protesting trucks on Tuesday also contained profanities against Ukrainian refugees, and Ukrainian officials said that the blockade had been fomented by far-right political figures.

“The issue is not with grain, but rather with politics,” President Volodymyr Zelensky of Ukraine said on Tuesday.

In recent months, Polish truck drivers have also blocked major crossings because of cut-rate competition from Ukrainian truckers, who are not subject to the same working hours and wage regulations as E.U. drivers. The Polish drivers have demanded that Brussels reinstate a permit system for Ukrainian truckers that was lifted after Russia’s invasion.

Edward Lucas, a senior adviser at the Center for European Policy Analysis, said that while the far right might be willing to exploit the protest to stir anti-Ukraine sentiment, the farmers’ grievances were legitimate, and that the Polish and European Union authorities should address them before they escalated into broader tensions.

“There is a risk that this is going to become geopolitically significant,” Mr. Lucas said, adding that Polish politicians “have allowed this to fester to the point that people are really quite cross.”