People are being urged not to attend "mass gatherings" that have supposedly been planned for city parks this weekend.
A number of posters have been shared online, with different locations including Cardiff's Bute Park and Heath Park, along with other cities around the UK.
It is still unclear if the posters are a hoax or a genuine protest against the coronavirus lockdown rules.
But people are being urged to avoid these locations for the safety of themselves and others.
The current lockdown guidance from the Welsh Government is that all gatherings of more than two people in public have been stopped, excluding people you live with, while parks will remain open for exercise but gatherings will be dispersed.
Authorities have also criticised posters that have advertised similar gatherings in Newport and Swansea.
Anna McMorrin, MP for Cardiff North, shared her concerns about the mass gathering to her Facebook followers, posting: "Please do the right thing - abide by the law, stay safe and protect lives."
Speaking to WalesOnline, she said: "I am deeply concerned by the prospect of mass gatherings in Heath Park on Saturday.
"I immediately reported this to South Wales Police who are taking the appropriate steps.
"The Welsh Government message remains the same - stay home.
"While this event may be a hoax, the damage may have already been done - encouraging those who seek to flout the rules that are there to protect us.
"The slightest change in our behaviour at present risks the virus spiralling and thousands of preventable deaths. I sincerely hope people will do the right thing, abide by the law and save lives.”
South Wales Police believe the gathering to be a protest but have urged people to take their protests online to avoid endangering lives.
A police spokesman said: "South Wales Police has always acknowledged the right to peaceful protest and regularly works with protest organisers to facilitate lawful and safe demonstrations which cause minimum disruption to the wider public.
"At the current time the overarching goal for all must be to stay home, protect our NHS, and save lives.
"We therefore urge anyone who wishes to protest to do so via online channels so as not to endanger themselves or others."
Councillor Peter Bradbury, Cardiff council's cabinet member for culture and leisure, said: “Holding an event in Cardiff’s parks requires permission from the council.
"Permission for this proposed ‘mass gathering’ has not been granted and anybody attending will be breaking the law if not socially distancing.
"This is a serious pandemic and we will not stand by and see people put their and other people’s lives at risk.
“We are aware that details of this unauthorised event have been widely shared on social media and would ask everybody to continue following the guidance in Wales to stay home, stay safe.”
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