The Operational Command in Warsaw made the announcement on Monday morning.
Polish Army officials said that over the past hours, there had been "stepped up activity of the long-range aircraft of the Russian Federation," designed to "carry out rocket strikes on targets located on the territory of Ukraine."
Poland's military has "activated all the necessary procedures devised to ensure the security of Polish airspace," and is "monitoring developments," the Operational Command in Warsaw said.
Russia launched a series of ballistic and cruise missiles on Ukraine in the early hours of Monday, The Kyiv Independent website reported.
Russian strikes injured three people in the northeastern city of Kharkiv and the surrounding region, according to officials.
There were also explosions in several other Ukrainian cities, including Dnipro, Krivyi Rih, Zaporizhzhia and Khmelntyskyi, Ukraine's authorities said.
As of 8 a.m. CET, the exact number of missiles fired by Russia and downed by Ukraine's air defences had not yet been announced, The Kyiv Independent reported.
On December 29, an air object temporarily entered Polish airspace, during Russia's large-scale air assault on Ukrainian cities that killed 39 people.
The Polish Army later said the object was a "Russian cruise missile" that then left Polish airspace.
Readings from Polish and NATO radars, as well as a subsequent ground search, confirmed that the missile had left Polish territory, officials said.
Monday is day 684 of Russia’s war on Ukraine.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
Source: PAP, Polish Army, The Kyiv Independent