About 60 vehicles were turned away from close to the base of Snowdon before 08:00 BST, police have said.
Cars were also towed after motorists were warned against parking illegally in Snowdonia National Park.
It followed chaotic scenes in the area last weekend when 180 penalty fines were issued.
North Wales Police said some drivers thought they had been “bluffing” with the threat of cars being towed and a recovery vehicle was at Pen-y-Pass.
The force said it was also monitoring the A5 at Dyffryn Ogwen and had placed cones along the route to prevent parking.
Pen-y-Pass car park, on the main A4086 road between Llanberis and Capel Curig, is only to be used as a drop-off site for buses and taxis at weekends in a bid to reduce car numbers.
“Around 60 vehicles have already been turned away this morning, with some even parking right by the signage,” said North Wales Police.
“The drivers told us they thought we were just ‘bluffing’ when we said cars would be towed away if obstructing the road… a recovery vehicle is already parked up at Pen-y-Pass.”
Traffic and parking in other parts of Snowdonia National Park is also being monitored and visitors have been urged to make use of car parks at Nant Peris, Llanberis and Pen-y-Gwryd as well as local park and ride services, which have been increased.
“This will help to control the traffic on these narrow mountain routes and avoid dangerous situations which we saw last weekend,” police added.
“We all want you to be able to enjoy the stunning mountain ranges safely but motorists must park responsibly, safely and legally.”
How bad is it elsewhere?
Wales’ national parks closed in March as part of coronavirus lockdown measures.
Walkers and hikers were allowed back when the “stay local” lockdown restrictions were eased on 6 July, while last weekend marked the start of the official school holidays for many visitors.
Brecon Beacons National Park has warned motorists civil enforcement officers are in the area to check for any parking illegally and to consider changing travel plans if car parks are full.
Pembrokeshire Coast National Park Authority has issued a warning over illegal camping after 19 penalty charge notices were issued last weekend around the county.
Wild camping, in a tent or campervan, is banned in Wales without the landowner’s permission.
Among the problems caused are littering, blocking access for emergency services and the lack of toilet facilities.
Campsites in Wales with shared facilities are now allowed to open, with strict rules and regulations, having missed most of the high season.
The National Park Authority said patrols will continue around the county this weekend.
“The majority [of visitors] were well behaved, however, there were several instances of illegal camping in the national park, which led to a number of fines being issued,” said chief executive Tegryn Jones.
“I urge people to respect the park and use designated campsites for your stay.
“We have longed for this time when we could enjoy access to this beautiful corner of Wales – let’s not let the irresponsible actions of a few spoil access and enjoyment for us all.”