BACKGROUND While community mobilization has been widely endorsed as an important component of HIV prevention among vulnerable populations such as female sex workers (FSWs), there is uncertainty as to the mechanism through which it impacts upon HIV risk. We explored the hypothesis that individual and collective empowerment of FSW is an outcome of community mobilization, and we examined the means through which HIV risk and vulnerability reduction as well as personal and social transformation are achieved. METHODS This study was conducted in five districts in south India, where community mobilization programs are implemented as part of the Avahan program (India AIDS Initiative) of the Bill & Melinda Gates Foundation. We used a theoretically derived "integrated empowerment framework" to conduct a secondary analysis of a representative behavioural tracking survey conducted among 1,750 FSWs. We explored the associations between involvement with community mobilization programs, self-reported empowerment (defined as three domains including power within to represent self-esteem and confidence, power with as a measure of collective identity and solidarity, and power over as access to social entitlements, which were created using Principal Components analysis), and outcomes of HIV risk reduction and social transformation. RESULTS In multivariate analysis, we found that engagement with HIV programs and community mobilization activities was associated with the domains of empowerment. Power within and power with were positively associated with more program contact (p < .01 and p < .001 respectively). These measures of empowerment were also associated with outcomes of "personal transformation" in terms of self-efficacy for condom and health service use (p < .001). Collective empowerment (power with others) was most strongly associated with "social transformation" variables including higher autonomy and reduced violence and coercion, particularly in districts with programs of longer duration (p < .05). Condom use with clients was associated with power with others (p < .001), while power within was associated with more condom use with regular partners (p < .01) and higher service utilization (p < .05). CONCLUSIONS These findings support the hypothesis that community mobilization has benefits for empowering FSWs both individually and collectively. HIV prevention is strengthened by improving their ability to address different psycho-social and community-level sources of their vulnerability. Future challenges include the need to develop social, political and legal contexts that support community mobilization of FSWs, and to prospectively measure the impact of combined community-level interventions on measures of empowerment as a means to HIV prevention.