Police are warning people thinking of visiting beauty spots over the warm Easter weekend that they will target those breaking rules around unnecessary travel amid the coronavirus pandemic.
A police and crime commissioner said forces will move to an "enforcement phase" rather than issuing advice.
A Welsh chief constable said some visitors were travelling along B-roads at night to evade police.
National parks have simply warned: "We're closed."
Dyfed-Powys Police and Crime Commissioner Dafydd Llywelyn said: "We are now getting to a position where the message is so clear that individuals who are out and aren't leaving the house for reasons stipulated in the guidelines, then there will be greater enforcement activity by the police."
The stark warnings come almost three weeks since First Minister Mark Drakeford announced a series of severe restrictions on people's lives to help stop the spread of Covid-19.
Mr Drakeford's announcement came just days after thousands of people flocked to Wales' beauty spots and national parks in scenes compared to a "bank holiday on steroids".
Good Friday and Saturday are set to be warm, according to BBC Wales weatherman Derek Brockway, with temperatures cooling on Sunday and Monday.
The official highest temperature in #Wales so far this year 22.2°C in Porthmadog on April 5th. We will beat that today with a high of 24°C predicted in the SE.— ᴅᴇʀᴇᴋ ʙʀᴏᴄᴋᴡᴀʏ ᴡᴇᴀᴛʜᴇʀᴍᴀɴ (@DerekTheWeather) April 9, 2020
Warm on Good Friday and Saturday but cooler on Sunday and Monday with a breeze@bbcwalestoday 1330 #StayHomeSaveLives pic.twitter.com/jpCsdCe9xX
What is being done?
Dyfed-Powys Police said it was putting extra officers on duty to catch people trying to flout the rules this weekend.
The patrols started on Thursday as the force stopped motorists on the A40 at St Clears, Carmarthenshire, the main arterial route into Pembrokeshire.
"We are still seeing people travelling in to Pembrokeshire and Ceredigion from as far afield as English counties," said Chief Constable Mark Collins.
"My message is: We are out there, patrolling.
"We'll be stopping vehicles and turning them around. I'm acutely aware there are people travelling in the hours of darkness, deciding to take B-roads. We'll be out on those B-roads."
Pembrokeshire council said it would help police catch those visiting second homes in the area, including those who arrive late at night to try and evade officers.
"We're paying attention where there are high density of second homes and holiday homes. We will have it covered," added Ian Westley, the authority's chief executive.
Pembrokeshire Coast National Park Authority said it had heard anecdotal evidence of a spike in visitors as early as Thursday, and told people to "stay at home".
Chief executive Tegryn Jones added: "The only way we can protect our rural communities, protect the NHS and save lives is to avoid travelling and to stay at home."
Brecon Beacons National Park Authority said its wardens would be out patrolling with Dyfed-Powys Police officers.
"If we continue to work together we will make a difference and I am sure people will continue to do so," added Julian Atkins, the authority's chief executive.
"Our National Park will still be here later for you to enjoy, so please make sure you and your loved ones are also still here to do the same."
In the past week, South Wales Police officers have responded to "a number of house parties, large family gatherings, football matches and groups of youths congregating", said Chief Constable Matt Jukes.
"Our officers and staff will continue to engage with our communities but with the restrictions now firmly embedded, the public can expect to see enforcement action taken against those who continue to flout the rules," he added.
"We don't want to issue fixed penalties or prosecute people, but we are being left with no choice due to the irresponsible actions of some."
North Wales Police is also increasing the number of officers on duty this weekend.
Officers were stopping traffic as it crossed the Britannia Bridge between the mainland and Anglesey on Thursday.
"Please do not travel to north Wales for the Easter weekend," the force said.
"Our tourist attractions, mountains, beaches, pubs, restaurants, caravan, holiday parks and campsites are all closed."
The force said on Good Friday it had arrested three campers near Wrexham after they ignored warnings to return home to the Wirral area.
Gwent Police is deploying additional officers this weekend, according to Chief Constable Pam Kelly.
She said despite an encouraging drop in the number of vehicles on the roads, the proportion of unnecessary journeys was still high.
"We are seeing almost 50% of the the people we are stopping are still taking journeys that are unnecessary," she said.
Urging people to stay home, she pleaded with people to think about those grieving the loss of loved ones to coronavirus.
"We are known for being kind and during this time - when many families are grieving - if there's anything people can do to support those people, or support those people who are unwell and support our NHS and wider community, it is to stay at home."