The 'European Alliance Against Depression' community-based intervention approach simultaneously targets depression and suicidal behaviour by a multifaceted community based intervention and has been implemented in more than 115 regions worldwide. The two main aims of the European Union funded project "Optimizing Suicide Prevention Programmes and Their Implementation in Europe" were to optimise this approach and to evaluate its implementation and impact. This paper reports on the primary outcome of the intervention (the number of completed and attempted suicides combined as 'suicidal acts') and on results concerning process evaluation analysis. Interventions were implemented in four European cities in Germany, Hungary, Portugal and Ireland, with matched control sites. The intervention comprised activities with predefined minimal intensity at four levels: training of primary care providers, a public awareness campaign, training of community facilitators, support for patients and their relatives. Changes in frequency of suicidal acts with respect to a one-year baseline in the four intervention regions were compared to those in the four control regions (chi-square tests). The decrease in suicidal acts compared to baseline in the intervention regions (-58 cases, -3.26%) did not differ significantly (χ2 = 0.13; p = 0.72) from the decrease in the control regions (-18 cases, -1.40%). However, intervention effects differed between countries (χ2 = 8.59; p = 0.04), with significant effects on suicidal acts in Portugal (χ2 = 4.82; p = 0.03). The interviews and observations explored local circumstances in each site throughout the study. Hypothesised mechanisms of action for successful implementation were observed and drivers for 'added-value' were identified: local partnership working and 'in-kind' contributions; an approach which valued existing partnership strengths; and synergies operating across intervention levels. It can be assumed that significant events during the implementation phase had a certain impact on the observed outcomes. However, this impact was, of course, not proven.