Using detailed daily information covering 100 countries and an event-study approach, we estimate the short run effects of implementing Non-Pharmaceutical Interventions (NPIs) on the spread of the COVID-19 virus at the early stages of the pandemic. We study the impact of two NPIs -stay-at-home requirements and workplace closures - on three outcomes: daily residential and workplace mobility; the daily growth rate of cases; and the daily growth rate of fatalities. Acknowledging that we observe a mobility reduction in countries before they implemented NPIs, we find that immediately after NPIs were implemented, mobility declined by 0.2 standard deviation (SD), and two weeks afterwards it was down by 0.7 SDs. 25 days after the NPIs were implemented, the daily growth rate of cases and deaths was lower by 10% and 8.4% respectively. Our results reveal that between 53 and 72 percent of the reduction of the daily growth rate of cases and deaths associated with a reduction of mobility is caused by NPIs.