A central issue in the context of smart cities is related to the capability to acquire timely information about city events. This paper describes a platform which focuses on processing messages posted in Twitter social network. Key issues here are the high throughput a large volume of data per second that needs to be processed, and the need to process ill formed natural language texts. With these in mind the platform has pipelined modules for robust, fast, real time tweet acquisition and storage, filtering of several kinds, natural language processing and sentiment analysis, that feed a final analysis and visualization module. A case study of sentiment analysis during the 2014 FIFA World Cup in Brazil is used to validate the effort made so far.