Recent advances in Structure-from-Motion and Bundle Adjustment allow us to efficiently reconstruct large 3D scenes from millions of images. However, acquiring the imagery necessary to reconstruct a whole city and not only its landmark buildings still poses a tremendous problem. In this paper, we therefore present an online system for collaborative city reconstruction that is based on crowdsourcing the image acquisition. Employing publicly available building footprints to reconstruct individual blocks rather than the whole city at once enables our system to easily scale to large urban environments. In order to map all partial reconstructions into a single coordinate frame, we develop a robust alignment scheme that registers the individual point clouds to their corresponding footprints based on GPS coordinates. Our approach can handle noise and outliers in the GPS positions and allows us to detect wrong alignments caused by the typical issues in the context of crowdsourcing applications such as malicious or improper image uploads. Furthermore, we present an efficient rendering method to obtain dense and textured views of the resulting point clouds without requiring costly multi-view stereo methods.