Fact checking by professionals is viewed as a vital defense in the fight\nagainst misinformation.While fact checking is important and its impact has been\nsignificant, fact checks could have limited visibility and may not reach the\nintended audience, such as those deeply embedded in polarized communities.\nConcerned citizens (i.e., the crowd), who are users of the platforms where\nmisinformation appears, can play a crucial role in disseminating fact-checking\ninformation and in countering the spread of misinformation. To explore if this\nis the case, we conduct a data-driven study of misinformation on the Twitter\nplatform, focusing on tweets related to the COVID-19 pandemic, analyzing the\nspread of misinformation, professional fact checks, and the crowd response to\npopular misleading claims about COVID-19. In this work, we curate a dataset of\nfalse claims and statements that seek to challenge or refute them. We train a\nclassifier to create a novel dataset of 155,468 COVID-19-related tweets,\ncontaining 33,237 false claims and 33,413 refuting arguments.Our findings show\nthat professional fact-checking tweets have limited volume and reach. In\ncontrast, we observe that the surge in misinformation tweets results in a quick\nresponse and a corresponding increase in tweets that refute such\nmisinformation. More importantly, we find contrasting differences in the way\nthe crowd refutes tweets, some tweets appear to be opinions, while others\ncontain concrete evidence, such as a link to a reputed source. Our work\nprovides insights into how misinformation is organically countered in social\nplatforms by some of their users and the role they play in amplifying\nprofessional fact checks.These insights could lead to development of tools and\nmechanisms that can empower concerned citizens in combating misinformation. The\ncode and data can be found in\nhttp://claws.cc.gatech.edu/covid_counter_misinformation.html.\n