The Polish military called the incursions an “act of aggression.” Poland and NATO air forces launched warplanes to respond, the Polish military said.

It was the first time that Russian drones had been shot down over the territory of a NATO country, Donald Tusk, Poland’s prime minister, said.

“We are dealing with a large-scale provocation,” he said. “The situation is serious, and no one doubts that we must prepare for various scenarios.”

It was not clear in the early aftermath exactly how many drones crossed into Poland. The Ukrainian Air Force put the figure at eight or more, part of a broader Russian attack overnight that sent 415 drones into Ukraine. The Polish military said the number of drones in its airspace was more than a dozen, though it did not say how many were shot down. Mr. Tusk cited a total of 19, according to Polish television.

Poland and NATO air forces launched warplanes and put ground-based air defenses on high alert, the Polish military said, adding that Dutch F-35 fighters had taken part in the response. There were no immediate reports of casualties in Poland.

Russia’s defense ministry did not immediately comment.

Russian drones have crossed into Poland before, including twice last week. But the apparent scale of the incursion and the joint NATO response in the early hours of Wednesday was a startling reminder of the risk that the war in Ukraine could escalate into direct confrontation between Russia and NATO. It was not yet clear whether Russia intentionally sent its drones into Poland, which would represent a clear expansion of the conflict.

“Increasing evidence indicates that this movement, this direction of strike, was no accident,” President Volodymyr Zelensky of Ukraine said, without detailing that evidence. “There have been previous incidents of individual Russian drones crossing the border and traveling a short distance into neighboring countries. But this time, we are recording a much larger scale and deliberate targeting.”

Even an accidental incursion, officials and experts say, could quickly spiral out of control. Poland and other Eastern European countries are already on high alert amid fears that the Kremlin’s expansionist designs could extend beyond Ukraine’s borders into the territories that once made up the communist bloc. Poland has bolstered military spending and has vowed to double the size of its armed forces to around half a million personnel. In May, Mr. Tusk openly flirted with the idea of acquiring nuclear weapons.

Mr. Tusk said he had informed NATO’s secretary-general, Mark Rutte, about what he described as “a huge number of Russian drones” that entered Polish territory.

Airspace was temporarily closed over parts of Poland because of the military operation, halting flights at some airports, including the country’s biggest, Warsaw Chopin Airport. The airspace over Warsaw Chopin was later reopened, but the airport said disruptions and delays would persist through Wednesday.

The Polish military warned people not to approach or touch wreckage from downed drones because it could be dangerous. It said the biggest threat was in Poland’s eastern provinces, including Podlaskie, on the border with Belarus; the province of Masovia, which includes the capital, Warsaw; and Lublin, which borders both Belarus and Ukraine.

Poles received an emergency alert on their phones Wednesday morning asking citizens to inform the authorities of any wreckage from falling drones. “Do not get close to them,” the alert said.

Rafał Jaworski, a taxi driver in Lublin, said the sound of fighter jets jolted him from sleep at around 2 or 3 in the morning.

“It looks like provocation,” he said. “A lot of airplanes, but at the moment it’s silence.”

Russia’s full-scale offensive in Ukraine is in its fourth year, and it has recently ramped up its drone and missile attacks on the country, despite efforts by the United States and other countries to broker peace.

On Sunday, Russia launched its largest drone assault so far in the war, the Ukrainian authorities said. After that attack, the Trump administration said it was prepared to increase economic pressure on Russia to get it to negotiate.

President Trump has grown increasingly frustrated with President Vladimir V. Putin of Russia, who has given little indication that he is ready to ease his maximalist demands to end the conflict.

Andrii Sybiha, Ukraine’s foreign minister, said on Wednesday that the incursions demonstrated the need to let neighboring countries use air defenses to intercept drones and missiles in Ukrainian airspace.

“Russian drones flying into Poland during the massive attack on Ukraine show that Putin’s sense of impunity keeps growing because he was not properly punished for his previous crimes,” Mr. Sybiha said.

Since Russia began its full-scale invasion in 2022, NATO countries, especially Poland, have been on high alert for any expansion of the Russian campaign into their territories.

“Repeated violations of NATO airspace by Russian drones are fair warning that Vladimir Putin is testing our resolve to protect Poland and the Baltic nations,” Senator Dick Durbin of Illinois, the No. 2 Democrat in the Senate, said on social media. “After the carnage Putin continues to visit on Ukraine, these incursions cannot be ignored.”