Question: When can we designate a specific masjid to be a "Masjid Ḍhirār"? And are prayers in such a masjid invalid?

Sheikh Muhammad Hisham At-Tahiri ḥafidhahullāh responds: "Certainly, designating a specific masjid as a "Masjid Ḍhirār" is not up to anyone. Why? Because this is a specific ruling, and specific rulings require a judge, a mufti, or a jurist. However, generally speaking, without specifying, scholars have mentioned the characteristics of "Masjid Ḍhirār." What are these characteristics? "Masjid Ḍhirār" are those built to divide Muslims. This is one of their signs. They are masjids where innovations are practiced, and the Sunnah is neglected. They are masjids where other than Allah is called upon and worshipped. These are the signs of "Masjid Ḍhirār." As for specific designation, for example, this is a central masjid where Friday prayers are held and people gather, if another masjid is built nearby intending to hold Friday prayers, it should not immediately be labeled a "Masjid Ḍhirār" just because it divides the congregation. The matter should be referred to the Fatwa Committee. If the Fatwa Committee issues a ruling that it is not permissible for the new masjid to hold Friday prayers, then prayers held there would be considered invalid, and Friday prayers there would not be correct. This is because the mufti, the Fatwa Committee, or the ruler in the country has prohibited holding Friday prayers in that masjid. Allah knows best." @almanhajussalafi