One man has already been rescued from Snowdonia today, while the British Mountaineering Council (BMC) warned that there may be no cover for some volunteer rescue teams because of the coronavirus crisis.
A man walking solo was rescued from the south ridge of Pen yr Ole Wen this morning.
Hundreds of cars could be seen filling car parks and on the side of roads in Snowdonia in pictures and videos posted to social media.
Closer to home, large numbers of people were seen at the Long Mynd in Shropshire.
It comes after the BMC warned that it would be "socially irresponsible" to take risks outdoors and tie up rescue services.
The BMC warned anyone braving the outdoors "do not assume that Mountain Rescue will be available" in what it called the "uncharted territory" of the coronavirus crisis.
In an article earlier this week the BMC said: "This is uncharted territory for all of us, and the advice we give to climbers, walkers and mountaineers will no doubt change over the coming days, weeks and even months.
"People need access to countryside for their health – both mental and physical.
"Follow the most current NHS advice regarding health and distancing. Currently Public Health England’s advice is 'you can go for a walk outdoors if you stay more than two metres from others'.
"Consider your means of travel and distance – close to home is best and, despite the environmental impact, it’s better to be in personal cars than public transport at the moment.
"Stick to familiar areas and low-risk activities.
"Reduce your risk. Be very aware that medical and rescue services and facilities are going to be extremely stretched and overwhelmed.
"It would be socially irresponsible to be taking risks at this time that could place an additional burden on medical and emergency services.
"Do not assume that Mountain Rescue will be available. There is a real possibility of reduced or even no cover for rescue in some areas as this develops – including along the coast that depends on lifeboat and volunteer coastguards."
Robin Millar, the MP for Aberconwy, criticised the "unthinking behaviour of visitors" and called for the British and Welsh governments to shut down holiday parks.
In a joint statement with David Jones, the MP for Clwyd West, Mr Millar said: "The government has been very clear that people should not undertake non-essential travel, but should, so far as possible, stay at home.
"Yet the roads of North Wales this weekend – the first weekend of Spring – have been full of people from outside the region making their way to holiday parks and cottages.
“These people may be touring from areas where coronavirus has already taken hold. Even if they are not themselves already displaying symptoms, they may already be infected and develop the infection or spread infection while in North Wales.
"Either way, they are liable to impose significant additional strain upon local health services, which are already overstretched. The North Wales Health Board has been in special measures for almost five years.
"The problem of temporary residents and the strain they impose on local health services is not confined to North Wales. Pembrokeshire, Scotland, Cornwall and North Yorkshire are other areas of the UK that are extremely concerned about the same issue.
“We are therefore calling upon the Department for Housing, Communities and Local Government in Westminster and the Welsh Government in Cardiff to take immediate concerted action to stop the use of holiday parks and holiday cottages for the accommodation of visitors for so long as the coronavirus outbreak continues.
“Such action will be beneficial not only to North Wales, but also to temporary visitors themselves. At a time of a national public health emergency, the last thing we need is people moving about the country, when the best medical opinion is that they should stay at home.”