Small groups with banners and Polish flags and joined customs officers checking trucks coming over the frontier from Lithuania, but made no attempt to block the Budzisko crossing, which is also used by vehicles carrying goods to and from Latvia and Estonia.
Farmers across Europe have been protesting for weeks against constraints placed on them by EU regulations meant to tackle climate change, as well as rising costs and what they say is unfair competition from outside the EU, particularly Ukraine.
The EU has waived quotas and duties on imports from Ukraine after Russia's invasion in 2022, angering farmers from neighbouring Poland, Hungary and Slovakia who say it undercuts their prices. Polish farmers argue that some Ukrainian grain which is brought to Lithuania is later re-exported to other countries as European grain.
"Today's protest is taking place to draw attention to this uncontrolled inflow of grain into the European market, grain coming from Ukraine, from Russia," protest organiser Karol Pieczynski told Reuters.
"It arrives in Lithuania, Latvia, documents are changed and those products are then distributed throughout Europe as a European product, which says it meets all standards, but it meets no sanitary standards." Lithuanian officials denied the accusations.
"I think people that are organising such events are mistaken, and this leads us into some escalation and some problems, logistical problems, between European countries, so it's quite sad," Vilmantas Vitkauskas, the head of the Lithuanian National Crisis Management Centre, said.
It wouldn't make commercial sense to bring Ukrainian grain to Lithuania and then re-export it to other countries, he added. The agriculture ministers of Poland and Lithuania released a joint statement urging farmers not to block the Budzisko crossing.
"They called on Polish farmers ... to allow the government institutions of both countries to complete investigations into grain trade flows between Lithuania and Poland," the statement read.
(Reporting by Janis Laizans and Anna Wlodarczak-Semczuk; Editing by Andrew Heavens and Crystal Chesters)