More than 100 people gathered in the Market Square area of Letterkenny at 2pm yesterday.
Gardaí confirmed they are looking to identify the organisers of the rally, adding that those in attendance are not under investigation as "social distancing, wearing of masks in public area and other such guidelines are non-penal regulations".
A Garda spokesperson said: "An Garda Siochana are carrying out an investigation to identify the organiser(s) of this demonstration.
"Gardaí are not investigating breaches of Public Health Regulations by those in attendance. Social distancing, wearing of masks in public area and other such guidelines are non-penal regulations.
"In supporting the COVID-19 public health guidelines and regulations, An Garda Síochána has and will continue to adopt, a graduated policing response based on its tradition of policing by consent. This has seen Gardaí engage, educate, encourage and, as a last resort, enforce.
"The COVID-19 Pandemic remains a public health crisis and An Garda Síochána continues to appeal to all citizens to demonstrate personal and social responsibility to comply with Public Health Guidelines and Regulations in order to continue to save lives."
Donegal has been placed at Level Three of the Government's Living With Covid-19 plan due to a spike in the virus infection rate in the county.
Up to six visitors from one household are permitted to visit another home, while bars, pubs and restaurants are only permitted to serve takeaway food, or outdoor dining for up to 15 people.
