The border guard agency posted a video of people camping and burning fires along the border fence, saying it was shot at a border crossing near the Belarusian village of Bruzgi.
"It is a new camp. There are still people living in the old one, a few hundred meters away. In total, there are about 1,000 foreigners in the area," it tweeted.
Hundreds of mostly Middle Eastern and African migrants have been amassing at the Belarusian border with Poland for weeks in an attempt to make it into the EU member state.
Polish Interior Minister Mariusz Kaminski said he would address the migrant crisis at a meeting with his German counterpart, Horst Seehofer, in Warsaw on Thursday. The growing migration pressure requires joint German-Polish action, he added.