The new rules took effect at midnight. Under the measures, Polish Border Guard officers, supported by police and soldiers, are authorized to stop and inspect selected vehicles.
The government said the checks are necessary to reduce irregular migration across the country's western ands eastern borders.
The controls will initially remain in place for 30 days, until August 5, authorities said.
Prime Minister Donald Tusk said last week that the move stems from the need to curb uncontrolled migration flows and respond to what he described as asymmetrical actions by neighbouring states.
He told a press conference last Tuesday that the decision was prompted by Germany's ongoing refusal to accept back migrants trying to reach its territory, a departure from past practice.
"This has led to tensions and a justified sense of asymmetry," the prime minister said.
"Without checks on our side, it’s difficult to verify whether the people being sent back to Poland should be returned at all," he added.
Germany introduced its own border checks with Poland in October 2023 in an effort to prevent illegal migration. These were initially meant to last until September this year, but Berlin recently decided to extend the checks, according to Tusk.
"Our response will be symmetrical," Tusk said. "The time when Poland failed to respond adequately to this type of action is over."
The prime minister insisted that Poland remains committed to the principle of borderless travel within the European Union, "but this must be a shared, symmetrical will among neighbours."
In the case of the border with Lithuania, Tusk said the reintroduced checks would apply not only to official crossings but also to so-called "green border" areas such as forests and local roads.
These operations will be supported by Poland's Territorial Defence Force (WOT) and other military units.
Anyone found crossing the Polish-Lithuanian border illegally will be returned to Lithuania "in accordance with the rules and regulations," Tusk said.
The prime minister acknowledged that the measures would also affect Latvia and Estonia to some extent.
He called on all three Baltic states, as well as the broader European Union, to work more closely with Poland to disrupt what he described as large-scale people-smuggling operations orchestrated by Russia and Belarus.
In recent weeks, Polish opposition politicians have accused German authorities of unlawfully transporting migrants into Poland and have shared such claims online.
Right-wing activists have begun organising so-called "citizen patrols" on the German border, attempting to inspect vehicles and posting videos of their actions.
Tusk called such behaviour "shameful and scandalous."
"Anyone who politically supports these kinds of activists—who disrupt the work of the Border Guard and even attack its officers—is undermining the state’s ability to ensure public safety," he said.
Opposition lawmakers, including Law and Justice (PiS) party leader Jarosław Kaczyński, have accused the government of being too passive.
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Source: IAR, PAP