The group has been protesting against what they claim was the centre's alleged attempt to "impose" Hindi in a largely Kannada-speaking state in the south. A campaign known as "Hindi beda" (we don't want Hindi) had also trended on social media earlier this month.
"At 5 in the morning our workers covered 40 metro boards, Hindi boards. Why are they forcing Hindi on us? If this continues we will stop the trains too," president of the pro-Kannada group Narayan Gowda told NDTV.
The metro authorities have said they have been following a three-language rule made by the centre. The state government, however, has found itself with little room to move since it shares the cost of running the metro with the centre in a joint venture, Bangalore Metro Rail Corporation Limited or BMRC.
The Kannada Development Authority had also issued notice to BMRC over the matter.
The activists, however, said they will continue to protest.
The Congress government in Karnataka has also set up a nine-member panel that will explore how the state can legally have its own separate flag and suggest a design, a move questioned not just by rival parties but also the Congress' leadership in Delhi.