Context. The atmosphere of Mars is characterised by a complex seasonal cycle of cloud formation related to the condensation of CO2 and H2O, and to the lifting of surface dust. Several decades of spacecraft observations have provided an impressive amount of data to constrain cloud properties. However, observations of a given cloud obtained from Mars orbit are typically limited in time sampling and spatial coverage. As a complement to this existing dataset, Earth-based telescopic observations have the potential to provide a global and dynamic view of some large-scale Mars clouds. Aims. On 17 November 2020, Mars and Earth were close to opposition. We took advantage of this configuration to attempt observing large-scale high-altitude atmospheric phenomena from Earth with a high time sampling, over several hours. Methods. Ten amateur astronomers were coordinated along with professional astronomers to observe Mars. Results. We observed the occurrence of a large-scale high-altitude cloud system, extending over thousands of kilometres from the equator to 50 S. Over 3 h, it emerged from the night side at 92 16+30 km and dissipated on the dayside. It occurred at a solar longitude of 316 (southern summer) concomitantly to a regional dust storm and west of the magnetic anomaly. Despite its high altitude, it was composed of relatively large particles (effective radius in the 1,2 μm range). While dust appears an unlikely candidate, possible composition by CO2 or H2O are both conceivable, although the whole properties of the cloud makes it atypical compared to previously reported clouds. We discuss the possible connections with the dust storm, along with the hypothetical role of nucleation from cosmic particle precipitation. Conclusions. We continuously followed a high-altitude huge cloud system on Mars from Earth, emerging from the Martian night, from its appearance at the terminator until its complete dissipation. It is either a large-grained water ice cloud system or an extended mid-summer dawn CO2 cloud system.