In one video, shared by the Belarusian blogging service NEXTA, migrants carrying rucksacks and wearing winter clothing were seen walking on the side of a highway.
Other videos showed large groups of migrants sitting by the road and being escorted by armed men dressed in khaki.
"Belarus wants to cause a major incident, preferably with shots fired and casualties: according to media reports, they are preparing a major provocation near Kuznica Bialostocka, that there will be an attempt at a mass border crossing," Deputy Foreign Minister Piotr Wawrzyk told Polish public radio.
Piotr Wawrzyk. Photo: PR24/MS
A deputy interior minister, Maciej Wąsik, tweeted that "Polish authorities are ready for any scenario".
The European Union has accused Belarus of encouraging thousands of migrants from the Middle East and Africa to cross into EU countries via Belarus, as a form of hybrid warfare in revenge for Western sanctions on Minsk over human rights abuses.
Belarusian President Alexander Lukashenko's government has repeatedly denied manufacturing a migrant crisis, blaming the West for the crossings and the treatment of migrants at the border.
The Belarusian state border committee issued a statement confirming that a large group of refugees was moving along the highway to the Polish border, and saying Warsaw was taking an "inhumane attitude".
The EU, the United States and Britain imposed sanctions on Belarus after Lukashenko unleashed a violent crackdown on mass protests following a disputed election last year.
Charities say the migrants face gruelling conditions while trying to cross the border from Belarus, enduring freezing weather and a lack of food, water and medical attention.
Polish authorities say at least seven dead migrants have been found on Poland's side of the border, with unverified reports of more deaths in Belarus.
Humanitarian groups accuse Poland's ruling nationalists of violating the international right to asylum by pushing migrants back into Belarus instead of accepting their applications for protection. Poland says its actions are legal.
Poland's Prime Minister Mateusz Morawiecki said on Facebook that "the Polish border is not just a line on a map. The border is sacred - Polish blood has been spilled for it!".
(Reporting by Matthias Williams in Kyiv and Joanna Plucinska in Warsaw; additional reporting by Pavel Polityuk; writing by Matthias Williams, editing by Ed Osmond)