Polish security forces say the move by Belarusian forces was the "most dangerous and serious so far".
Poland's foreign ministry said it has summoned a Belarusian diplomat over an "intrusion" at the country's border.
Poland says that "unidentified uniformed individuals armed with long guns" crossed into its territory on Monday night.
The foreign ministry has described the incident as a "provocation" and said it had summoned the Belarusian chargé d'affaires, Alexander Chesnovsky.
The incident comes as Poland faces significant migration pressure on its eastern border with Belarus.
Stanislaw Zaryn, a spokesman for Poland’s security services, said the incident was "the most dangerous and serious so far".
“After meeting a Polish patrol, they reloaded their weapons and then departed towards Belarus,” Zaryn said. The incident occurred around 300 metres from the Belarusian border, he added.
Poland's deputy foreign minister, Piotr Wawrzyk, has demanded an explanation and stated the Belarusian officials risk escalating tensions.
"The actions taken by the Belarusian authorities in recent weeks have the increasingly evident hallmarks of a deliberate escalation," he said in a statement.
"Poland deems such actions unacceptable and will not tolerate them ... Poland is determined to defend its borders and the external borders of the European Union," he added.
In recent months, thousands of migrants from Syria, Iraq, Afghanistan, and Africa have entered Poland and other EU member states, many carrying Belarusian tourist visas.
On Thursday, the Belarusian Foreign Ministry, in turn, summoned the Polish charge d'affaires to express its "strong protest".
"Belarus has never caused any border incident and therefore categorically rejects" these allegations, the ministry said in a statement, adding that "the deterioration of the situation at the border" was "unacceptable".
The European Union has accused Minsk of deliberately encouraging illegal migration in retaliation against EU sanctions, in an effort to destabilise the bloc.
A number of migrants and refugees remain stranded between the two borders in increasingly difficult autumn weather conditions.
At least ten people have died so far in the region, including an Iraqi migrant found on the Polish side of the border on Friday.
Poland has sent thousands of soldiers to the border and enforced a state of emergency in the region to combat the increase in migrant arrivals.