Cities account for 75% of carbon emissions today. Achieving global emission goals requires city leadership. Rapid and radical decarbonisation, however, can only be sustainable if it is socially just. Transformations that go beyond technical solutions require holistic approaches and buy-in from diverse stake-holders and citizens. To trigger such processes in a highly urbanised region, catalyst projects in five Urban Labs in Mexico, Brazil and Argentina will be implemented and new models of coalition building and governance tested. This process is enhanced by transformative research, tailored communication strategies and capacity building activities. The project will also utilize film and art to inspire new narratives of attractive urban futures and shift mind-sets towards sustainability. Finally, models of coalition building and governance will be scaled up to other cities and countries, and ultimately to influence global agendas for sustainability transformations.