Several anti-racist demonstrations took place around the UK and the world following the death of George Floyd while in police custody in the USA in May 2020. During the incident Mr Floyd could be heard saying 'I can't breathe', hence several placards at the demonstrations citing these words.

From blacklivesmatter.com: Black Lives Matter Foundation, Inc is a global organization in the US, UK, and Canada, whose mission is to eradicate white supremacy and build local power to intervene in violence inflicted on Black communities by the state and vigilantes.

The reason many attendees were wearing face masks was because the events took place during the worldwide Covid-19 pandemic. Despite government regulations limiting gatherings to just 6 people, large scale events were held.