Citizen-generated populism is flourishing in the comments sections of online news. The factors that shape the extent of such populist communication from below are still under-researched. This study focuses on the COVID-19 crisis to examine how contextual and media-related factors are related to the extent of populism in comment sections on Facebook pages of news outlets from seven European countries (AT, DE, FR, IT, NL, SE and UK). Computational text analysis, machine translation and Bayesian multilevel regression were used to analyze digital trace data from 65,258 posts and 3.4 million comments published between February 2020 and June 2021. The computational measurements - multilingual dictionaries for posts and distributed dictionary representation to capture populism in comments - were rigorously validated. The results show that posts referring to the government, experts, COVID-19, and restrictions exhibit higher levels of populism in the comments sections. The stringency of containment policies was positively associated with populism in Germany, Austria, and the Netherlands when COVID-19 was mentioned. Lower levels of populism were observed for tabloid media and when news outlets engaged in visible moderation. The implications of these findings beyond the pandemic context and methodological challenges are discussed.