West Bengal Chief Minister Mamata Banerjee on Monday branded the Goods and Services Tax (GST) a "Great Selfish Tax" and said it was being used to harass people and destroy the nation's economy.She also termed the demonetisation a disaster and urged Twitter users to change their display pictures to black to protest against the note ban."Great Selfish Tax to harass the people. To take away jobs. To hurt businesses. To finish the economy. (The government) totally failed to tackle GST," Banerjee tweeted."Note ban is a disaster. On November 8 Black Day, to protest against this scam that destroyed the economy, let us also change our Twitter DP (display picture) to black."Banerjee changed the display picture of her Twitter account to black on Monday.The Trinamool Congress chief had earlier instructed her party leaders to observe "Black day" against demonetisation on November 8 to mark its first anniversary.