Large-scale single-cell analyses have become increasingly important given the role of cellular heterogeneity in complex biological systems. However, no techniques at present enable optical imaging of uniquely tagged individual cells. Fluorescence-based approaches can distinguish only a small number of distinct cells or cell groups at a time because of spectral crosstalk between conventional fluorophores. Here we investigate large-scale cell tracking using intracellular laser particles as imaging probes that emit coherent laser light with a characteristic wavelength. Made of silica-coated semiconductor microcavities, these laser particles have single-mode emission over a broad range from 1,170 nm to 1,580 nm with sub-nanometre linewidths, enabling massive spectral multiplexing. We explore the stability and biocompatibility of these probes in vitro and their utility for wavelength-multiplexed cell tagging and imaging. We demonstrate real-time tracking of thousands of individual cells in a three-dimensional tumour model over several days, showing different behavioural phenotypes.