Video footage showed hundreds of people near a barbed-wire border fence, which some tried to force their way through.
The Polish government called a crisis meeting on Monday and deployed 12,000 troops to the region.
Poland has accused Belarus of pushing the migrants towards the border, describing it as hostile activity.
Poland, Lithuania and Latvia say there's been a surge in the number of people trying to enter their countries illegally from Belarus in recent months. Many of them have come from the Middle East and Asia.
The European Union has accused Belarus's authoritarian President Alexander Lukashenko of facilitating the influx in retaliation against sanctions.