Poland's Border Guards say the alleged instance of violence was "staged" by activists.
A Polish MEP has been accused of lying about alleged violence committed by border guards against migrants.
Janina Ochojska, the founder of the Polish Humanitarian Action group and the European Parliament member from the centre-right Civic Platform, is facing legal action over her comments.
Last month, Ochojska shared a tweet accusing Polish border guards of violently pushing migrants back across the border to Belarus.
Ochojska said she had herself witnessed border guards forcing some activists to lie and kneel on the wet ground in the border region.
"Only our presence and our cameras ... saved migrants from another deportation," she wrote on Twitter.
But Polish officials have said that the alleged incident was staged and Poland's deputy interior minister Maciej Wąsiklater confirmed on Twitter that the MEP would be sued for insulting a public office.
"We will not allow her public comparisons to be addressed to the Border Guard," Wąsik told state television last week.
"[These statements] are not true, not legitimate [and] political," he added.
The Polish Border Guard has repeatedly denied allegations of excessive force and illegal pushbacks at its border with Belarus.
Spokesperson Anna Michalska said the alleged incident referred to by Ochojska had been deliberately filmed to "record how we deal with foreigners."
"The worst thing was that instead of working with us — we always say that we are open to working with foundations — these people obstructed our work."
A group of citizens has also said they would submit complaints against the Regional Prosecutor's Office in Warsaw in solidarity with Ochojska.