Barry Island was described as being as "busy as a summer's day" despite the Level 4 stay local restrictions in place in Wales.
There were reports of traffic queues getting into the town's car parks and long lines of customers at the takeaways after the sun came out on Sunday.
Boxing Day was wet and windy, but it was sunnier on Sunday and coastal areas, including Barry Island and Cardiff Bay were reported to be extremely busy.
Wales' First Minster Mark Drakeford announced that Level 4 lockdown restrictions would start in Wales on Saturday, December 19.
It means people must stay at home, except for very limited purposes, while all non-essential shops, gyms and hospitality must close. What the restrictions are.
One local resident said: "Despite Wales being in lockdown, Barry Island is full of visitors.
"Cars can be seen queuing on the causeway and the new road, car parks are filling up and there are queues outside any takeaways.
"It seems unbelievable that people are clearly disregarding the requirement to stay home, and using the 'exercise' element as an excuse for a 'jolly', with no consideration for others or the residents here.
"Although social distancing is happening, more by chance than design, others stand in the middle of footpaths totally oblivious to those needing to pass safely."
Another said on social media: "The place is rammed. Very little social distancing."
And one added: "Just drove past Barry island and it’s the busiest I’ve seen it since the summer. Absolutely crazy, go home!"
Latest figures from Public Health Wales published on December 27 show 4,142 new cases of the virus have been recorded to bring the total since the pandemic began to 139,642.
The overall death total now stands at 3,368 in Wales. Cases in your area here.
Following the latest figures, the infection rate is now 558.6 cases per 100,000 people based on the seven days up to December 22.
