Poland introduced temporary controls on its borders with Germany and Lithuania on Monday in an effort to stem what the government says is an increasing number of undocumented migrants crossing from the north and west.

The re-imposition of border checks is just the latest example of how mounting public concerns across the European Union over migration are straining the fabric of the bloc's passport-free Schengen zone. The Netherlands, Belgium and Germany have already implemented similar measures.

In Poland, the debate over migration has become increasingly heated in recent weeks, with groups of far-right activists launching "citizens' patrols" on the western border amid Polish media reports of German authorities sending undocumented migrants back across the frontier.

"Everything is proceeding without incident," Interior Minister Tomasz Siemoniak told private broadcaster TVN24 on Monday after the controls came into effect.

"Traffic is currently moving smoothly, 800 police officers, 200 gendarmerie soldiers, 500 territorial army soldiers," Siemoniak said, adding that all services were "in full readiness."

Speaking at a midnight press conference on the German border, when the checks began, Siemoniak said only state officials, such as border guards, were authorized to check vehicles entering Poland, referring to the "citizens' patrols."

The border guard said in a post on X that it had detained an Estonian citizen on Poland's border with Lithuania for transporting four illegal migrants believed to be Afghans.

Hardening public mood

Public sentiment in Poland toward migrants has hardened since a 24-year-old woman was killed in the city of Torun by a Venezuelan citizen in June. On Sunday, some 10,000 people took part in a march organized by nationalist activists in remembrance of her.

Meanwhile, on Saturday evening, a Polish man died after being stabbed during a fight in Nowe, northern Poland. A Colombian citizen was arrested on suspicion of being responsible.

Police said on Monday that they had detained a total of 13 people in connection with the incident – three Poles and 10 Colombians.

State news channel TVP Info showed angry crowds gathering in the town outside the workers' hostel where the Colombians lived.

Human rights activists condemned the "citizens' patrols."

"The actions of these self-proclaimed groups are the result of a radicalizing political narrative that presents migration as a threat, which fuels social fears and distrust of state institutions," the Helsinki Foundation for Human Rights said in a statement on Friday.

"The Foundation once again appeals for an honest and reliable public debate on the migration situation and border policy, based on facts, not fear and manipulation."