Vandals smashed a dozen windows at the Muslim Cultural Centre in Warsaw, which houses one of the two mosques in the Polish capital, police said Monday.
"We didn't find any writing on the building and for now the case is being treated as property damage," said police spokeswoman Edyta Wisowska.
The incident comes after previous threats and a prior act of vandalism on the building, amid an atmosphere of intolerance that has grown since Poland's right-wing government took power in 2015.
The vandals also damaged the frame of a large tent erected in the centre's garden.
Police were called to the building in the early hours of Monday.
Centre coordinator Ahmad Alattal said anti-Muslim hate speech has been on the rise in Poland in recent years, especially since the start of Europe's migrant crisis.
"We receive threats all the time by e-mail, on social media and also by telephone," Alattal added. "A month and a half ago we had a similar, but smaller incident. A glass bottle was broken against a window on the front door."
"The only response we have received from the Polish authorities was from the Senate Chancellor saying that it understood our fear but that our worries were unfounded," he added.
At a massive far-right march earlier this month in Warsaw some of the participants chanted "Pure Poland, white Poland" and "Refugees get out".
The leader of the ruling Law and Justice (PiS) party, Jaroslaw Kaczynski, campaigned on a staunchly anti-refugee platform in 2015, claiming that refugees were bringing "cholera to the Greek islands, dysentery to Vienna, various types of parasite".
