The 2.5-metre (8.2-foot) high fence is intended to run 180 kilometres -- nearly half the total length of the forested border between the two countries.
Dozens of soldiers could be seen working on the fence on Friday near the border town of Krynki, where residents were divided about the new barrier.
Thousands of migrants -- mostly from the Middle East -- have crossed or attempted to cross from Belarus into the eastern EU member states of Latvia, Lithuania and Poland in recent months.
The European Union suspects that the flow of migrants is being directed by the Belarusian regime as a form of retaliation against EU sanctions.
Poland and the Baltic states have called it a "hybrid attack" on the entire EU and have vowed to seal the border to prevent migrants getting in, leading to several stand-offs.
Polish border guards on Friday said they had repelled 3,059 attempts to cross the border so far in August, including 55 in the last 24 hours.
Poland in recent days has also doubled to 2,000 soldiers its military contingent along the eastern border to assist the border guard service.
A group of migrants, who according to a charity helping them are all from Afghanistan, has been stranded at the border for nearly three weeks.
Polish guards are preventing them from crossing to make asylum claims, with the government saying that this would be giving in to the "blackmail" of Belarusian strongman Alexander Lukashenko.
The UN refugee agency, the Council of Europe and the European Court of Human Rights have all urged Poland to provide humanitarian aid to the migrants.
But Poland has said it can only provide assistance with the consent of Belarusian authorities.
Polish officials say the migrants are camped out just a few centimetres into Belarusian territory.
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