Increasing amounts of litter could have a "devastating" effect on health, wildlife and tourism in Wales, an environment charity has warned.
Crowds of people have been gathering in public spaces since lockdown measures were eased in Wales.
However that has seen some beauty spots "treated like a rubbish tip" and tourist areas spoilt by litter.
Volunteers said they are struggling to cope and councils have warned cleaning the waste is taking up resources.
Keep Wales Tidy said there is now a "clear and present issue with increasing amounts of litter across Wales" as crowds flock to beaches, parks and the countryside, as seen in Ogmore-by-Sea on Thursday.
At the same time, large crowds gathered in Roald Dahl Plass in Cardiff Bay, leaving behind empty bottles, cans and food containers.
The images of rubbish scattered in visitors' wakes have sparked outrage and resulted in pleas by local authorities and residents, as well as broadcaster and naturalist Iolo Williams to either take rubbish home or stay away.
Cardiff council said it understood why people wanted to spend more time outdoors but urged them to take greater responsibility for their own litter.
"What isn't acceptable is for residents to leave their litter in parks and open spaces, ruining them for everyone else, and expecting someone else to clear up the mess they've left behind," a spokesman said.
We want all visitors to be able to enjoy the beauty of #CardiffBay.— Visit Cardiff Bay (@VisitCardiffBay) June 25, 2020
Please don’t treat it like a rubbish tip and ruin it for others – put your litter in the bins provided, or if they’re full, take it home with you. #DontBeALitterBug #KeepItClean @Keep_Wales_Tidy https://t.co/DFdyypBRxY
Photos also appear to show litter scattered across beauty spots in Llanberis, Gwynedd, while others taken on the Great Orme above Llandudno, Conwy, were said to include human waste.
Conwy council said it has seen littering returning since lockdown laws were relaxed.
With the message changing from "stay home" to "stay local", people are able to spend time outdoors for activities, although this should be within five miles of your home as a "rule-of-thumb".
It means you are able to have a picnic or a barbecue, with people from one other household, as long as you stay socially-distanced from them and others and follow strict hygiene rules.
"There's no excuse for littering. Most people dispose of their litter responsibly, and it's disappointing that there's a minority of people who don't," said Conwy county councillor Greg Robbins.
Anti-litter campaigners in Monmouth said they were struggling to cope with the "huge quantities" of litter left on the banks of the River Wye and have appealed for residents' help.
Volunteer Emma Bryn said: "We are so lucky to live in such a stunningly beautiful part of the country, which we share with some amazing wildlife. Littering here is thoughtless on so many levels."
Meanwhile, Monmouthshire council staff have referred the problem to Gwent Police after nitrous oxide capsules were recovered.
Powys council said it saw problems within hours of a McDonald's drive-through reopening in Newtown, with bins overflowing, and issued a fine for littering.
It added: "Street cleansing across the county has continued throughout the pandemic and, as restrictions ease, we will keep up monitoring and step up our awareness and enforcement activity around litter."
However charity Keep Wales Tidy said caring for the environment was not the sole responsibility of local councils.
"If we learnt anything from the hugely popular Blue Planet series it is that unless we take action on this now then we face a devastating impact on our tourism, our health and our wildlife," said chief executive Lesley Jones.
"Everybody needs to take collective responsibility right now to stop this from happening. People need to take their litter home. This one simple change will make a big difference."
Meanwhile, Glandwr Cymru has appealed for help cleaning canals in Wales with its usual volunteering activities still on hold since the start of the coronavirus epidemic.
And the National Trust Wales and Marine Conservation Society cited "significant antisocial behaviour" such as littering, particularly fast food waste, with lockdown restrictions being eased, to the Senedd Climate Change, Environment and Rural Affairs Committee hearing this week.