The activists of the Raj Thackeray-led outfit went around Louis Wadi area of Thane city last night, blackening boards of shops which were displayed in languages other than Marathi.
They also warned shopkeepers to immediately change their boards to the Marathi language in addition to other languages.
Meanwhile, Thane Mayor Sanjay More presided over a function organised by the Thane Municipal Corporation to mark the 'Marathi Rajbhasha Divas'.
Several other functions were organised in the city to celebrate the occassion.