Wales’ first town-only lockdown will come into force later.
Llanelli in Carmarthenshire will have restrictions imposed from 18:00 BST, making it the first town hit with restrictions without the rest of the county it belongs to.
Wales’ two biggest cities – Cardiff and Swansea – will follow suit 24 hours later following Covid-19 spikes.
Health Minister Vaughan Gething told BBC Breakfast the situation was “very serious” and comparable with February.
“We ended large parts of NHS activity about two weeks later. We had a full lockdown three and a bit weeks later,” he said.
“We’ve seen a sharp rise in cases in all the areas we’re taking local restrictions and it’s being driven by indoor household contacts, so more people than should be in that household going in and mixing.
“And that’s extending out into licensed premises as well where, again, people aren’t following the rules.
“You can go out for a drink but only with people in your household bubble.”
People in 13 ward areas in Llanelli cannot leave town, or mix indoors with anyone outside their own household.
The town has seen 85 coronavirus cases over the past week – compared to 24 across the rest of Carmarthenshire.
“It is worrying to see how sharply the number of positive cases has risen in the Llanelli area and action has had to be taken to help stop the spread and break the chain of infections concentrated in this area to prevent a whole county lockdown,” said Carmarthenshire council leader Emlyn Dole.
“Through the test and tracing we can track the rise in cases back, in some instances, to establishments maybe who haven’t been paying as much heed to the restrictions as the rest of us to social distancing, and to ensuring that social distancing is kept in places where the public meet.
“That, I guess, has led to the spike in cases,” he told Radio Wales Breakfast.
The rate of infection across the town has leapt to 152 cases per 100,000 of the population – it is just 18 per 100,000 for the rest of the county.
It places Llanelli in the top three weekly infections rates across Wales, alongside Blaenau Gwent on 202 per 100,000 and Merthyr Tydfil at 169 per 100,000.
Neighbouring Swansea hit a seven-day rate of 56 per 100,000 on Friday, while Cardiff reached 38 cases per 100,000.
The two areas will face the same additional coronavirus restrictions as Llanelli from 18:00 on Sunday.
Chairwoman of Hywel Dda University Health Board, Maria Battle, said: “Our local community has given us such tremendous support during the past few months.
“To protect the health of our people, including the most vulnerable, and to ensure our NHS resources are available to provide people with the care they need, we need the help of our Llanelli population and wider community now more than ever before.”
Extra testing capacity is being introduced, with appointments at Parc y Scarlets, Ty’r Nant at Trostre, and the Carmarthen showground.
Health officials said there should be “no reason for Llanelli residents to travel excessive distances for a test”.
Cardiff and Swansea go into lockdown from Sunday evening.
Cardiff Central Labour MP Jo Stevens said it was “inevitable” given the capital city was surrounded by council areas already in lockdown.
“I would reinforce what the Health Minister Vaughan Gething said yesterday, you know, don’t take this weekend to go on a massive bender,” she told Radio Wales Breakfast.
“It’s not going to be helpful.
“If people do that this weekend, it does risk further infection rates and that means we’ll be in local lockdown for longer.”
Thomas Beynon, manager at Three Cliffs Bay Holiday Park, Gower, said he was expecting to cancel about 380 bookings before the season ends in November due to the new lockdown in Swansea.
He said it was “hugely deflating” and meant “strange times again” after the business was hit by the national lockdown earlier this year.
He said customers had been “very supportive” by transferring bookings to next year rather than cancelling and seeking a refund.
“We are extremely humbled,” he said.