Background Programmes for collective smoking cessation, based on the British model Stoptober, are proposed by public health units in many countries. There is a need for data estimating the rate at which participants in these programmes are successful in quitting smoking. We report a prospective study carried out as part of a large-scale collective cessation programme conducted in Switzerland in 2017.Methods 1112 participants among the 7008 smokers enrolled in the collective cessation programme were recruited before the start of the attempt. Continuous abstinence was measured 10 days, 3 months and 6 months after the start of the attempt. Participants who dropped out at follow-up were considered to have failed the attempt (worst-case scenario).Results The continuous abstinence rate was at least 37.9% at 10-day follow-up, 18.8% at 3-month follow-up and 13.1% at 6-month follow-up. Similar levels of continuous abstinence as the worst-case scenario were found in sensitivity analyses including those whose quit attempt started before the beginning of the programme and where multiple imputation was used to replace dropouts. Sensitivity analyses using complete cases or an indicator of abstinence which allows occasional lapses found around double the abstinence rates.Conclusions Our results support the potential usefulness of large-scale collective cessation campaigns and suggest that such programmes based on social networks are promising areas for future smoking cessation programme activity.