Prosecutors are investigating after a 31-year-old man allegedly placed powerful explosives in a busy, central area of Warsaw.
The suspect had reportedly taken a device out of a backpack and placed it on the pavement on Monday.
The incident occurred near the Polish presidential palace and Warsaw's Old Town tourist area.
More than 300 people were taking part in a commemorative march for victims of a World War II massacre when police detained the man and evacuated the area. A police bomb squad then removed the device.
Authorities were waiting for expert opinion on the type and potential range of the explosive device, according to Aleksandra Skrzyniarz, a spokesperson for the city's prosecutors’ office.
The suspect was questioned on Wednesday on suspicion of terrorism, Skrzyniarz told reporters. Due to the seriousness of the potential changes, the National Prosecutor’s Office has taken over the investigation.
The suspect could face up to five years in prison. Police say he is known to them and has a "previous criminal record".
Authorities initially described the situation as “serious,” but there was no explosion and there were no reports of anyone being hurt.