Sussex Police said they were made aware of a gathering of around 150 people in Brighton's Valley Gardens shortly before 6pm yesterday (March 13).
Reclaim These Streets vigils had been organised in memory of 33-year-old Sarah, whose death last week sparked national outcry around women not feeling safe on the streets.
The event was officially cancelled after Sussex Police warned people not to gather, but vigils still popped up across the country.
A spokesman for Sussex Police said officers arrived and encouraged gatherers to leave and, when they did not comply, moved to 'necessary and proportionate enforcement action'.
"An 18-year-old man was arrested at the scene on suspicion of failing to comply with a direction in order to control coronavirus," said the spokesman.
"He has been taken into custody. Eight Fixed Penalty Notices were issued to people for breaches of the government’s coronavirus regulations.
"We recognise the desire to come together at this time, to mourn the death of Sarah Everard in London and to make a statement on the issue of women’s safety, and we absolutely understand the importance of this message. Everyone should feel safe to walk our streets and we hear that many of you do not.
"We acknowledge many women have experienced violence, abuse or harassment and this is not acceptable. Protecting our communities is a top priority for Sussex Police, and we will do what it takes to ensure you feel safe in the county where you live.
"However, we remain firmly in a public health emergency and the Covid-19 regulations continue to disallow large gatherings because of the continued, and very real, risks of the virus. A High Court judgement on Friday (March 12) ruled planned vigils would not be an exception to the government’s legislation around gatherings during the national lockdown.
"We are grateful to everyone who has found a safe and alternative way to express their feelings at this difficult and emotional time."
Demonstrators also gathered in Shoreham's Coronation Square last night.