Last week, a Zoom meeting organised by the 40,000-strong Federation of Young European Greens was infiltrated by someone streaming child sexual abuse material.
It was reported to police in Belgium, where the organisation is based.
A similar incident is also believed to have happened last Friday during an online discussion about Covid-19 contact tracing apps, hosted by the Open Rights Group, which aims to “preserve and promote” people’s rights in the digital age.
The Internet Watch Foundation, which works to remove child abuse content from the web, said it had been alerted.
Zoom said it was “looking into” what had happened.