Poland's Law and Justice ruling party has sought to downplay the scandal as it fights for an unprecedented third consecutive term.
Marcin Kierwinski, the secretary general of the Civic Coalition electoral alliance to which the 33-year-old Gajewska belongs, denounced the detention.
"These are Belarusian standards," Kierwinski said. "The police detained her even though she repeated that she was a member of parliament."
Police issued a statement saying they had not been aware that she was a lawmaker.
A video posted by Donald Tusk, the main opposition leader, shows police putting her into the van even as people told police that she is a lawmaker.