Demand across England's High Streets, retail parks and shopping centres surged on Monday as some shops reopened after a three-month lockdown.
Research firm Springboard said that by 17:00, footfall was 38.8% higher than last week, as pent-up demand led to reports of long queues.
However, shopper numbers were generally far below the same time last year.
But at Bicester Village, near Oxford, crowding was so great that 3,000 people signed a petition to close it.
All shops in England are now allowed to open, but with strict safety measures.
"Footfall has risen by more than we anticipated, but it was certainly helped by the weather, which made queuing a more pleasurable experience," said Diane Wehrle, marketing and insights director at Springboard.
Safety measures include plastic screens at the tills and floor markings to keep shoppers two metres apart - measures that are already a regular fixture in supermarkets.
Jaydee Darrock, from Warwickshire, was one of the first inside Primark's Birmingham store after queuing from about 07:00. She described it as "surprisingly calm".
"We thought it would be chaotic, it's not, it's quite nice," she said.
There were also big queues outside the Nike store in Central London, although some complained there had been a lack of social distancing.
In Manchester people waited for almost an hour outside Primark, TK Maxx and Foot Locker, although elsewhere demand was more subdued.
Although food shops, pharmacies, banks and other essential retailers have stayed open, vast swathes of the High Street, from bookshops to clothes outlets, have been closed since 23 March.
Springboard said footfall was likely to rise further in the coming weeks, but warned many shoppers still had concerns about other people not following the safety measures in stores and there being long queues to get in.
What are people shopping for?
Shantel Brown, 35, was shopping in Milton Keynes with her daughter Tee, who is 16.
Shantel said: "I bought nightwear and baby clothes because Primark was open. I'm pregnant and I've been waiting to get some baby stuff.
"Everyone keeps their distance. They've got sanitiser at the entrance. As long as we've got our masks on we're fine."
Tee added: "I bought tops, shorts and summer clothes. I came because Mum forced me."
Greg Dulson, 68, was shopping for a new watch strap.
"The strap on my favourite watch broke and I brought it in to the watchmaker's the day before lockdown," he said.
"They said come back tomorrow, but it was closed!"
Katie Kirby, who is 18, was shopping with her 21-year-old boyfriend Zac Hopkins.
"I bought a jumpsuit because Primark was open and the weather's getting better again," said Katie. "I did go just to get some essentials like pants and socks, but I when I saw the jumpsuit I had to treat myself because the shops haven't been open for so long."
"We do keep our distance in the store. We thought it would be a different shopping experience but once we were in there it was just the same."
Zac Hopkins added: "I bought a skipping rope, so I can do some exercise at home."
'It's rather normal'
Business reporter Vivienne Nunis in Milton Keynes
There were socially distant queues outside Zara, Primark and TK Maxx this morning and a steady crowd of shoppers are now browsing in the centre. Lots of young people are here, mothers with strollers and some older shoppers too. Some people are wearing masks, but not many.
However, lots of stores remain closed and all the lights are off in the locked cafes and fast-food restaurants.
The shoppers are mostly sticking to the new rules and staying socially distant - when they remember. Floor markings indicate a one-way system, every second toilet cubicle is closed, touch-screen maps have been powered down and seating areas are roped off. But apart from those changes - and a growing queue outside Primark - everything feels rather normal.
HMV owner Doug Putman told the BBC's Today programme that he expected a rush in the first week of trading after his shops open their doors. But he said retailers could be faced with a problem if shoppers do not return in the same numbers as before the lockdown.
"If you've got the same cost structure to run the business but sales are down even 20% it makes a lot of companies unviable.
"We're being very hesitant, we believe that it is going to be a tough year."
Other safety measures brought in by retailers include pleas not to touch items unless customers are thinking about buying them and decontaminating shopping baskets after each use.
Shops are also promising there will be plenty of sanitiser on hand for customers. Toilets will remain closed in Primark's shops but facilities in other stores, including John Lewis and Selfridges, will be open.
In most clothes shops, fitting rooms will be closed. Bookshops including Waterstones intend to put items in quarantine if browsed but not bought, while some jewellers are introducing ultraviolet boxes that can decontaminate items in minutes.
Government guidance suggests shops put some items aside if they have been extensively handled by customers.
Not all stores will throw open their doors immediately. John Lewis will reopen just two outlets on Monday - in Kingston upon Thames and Poole - as part of a phased approach, with 11 others to follow on Thursday.
Major High Street chains start to unlock
- John Lewis will start by opening 13 branches in stages
- Next will reopen just 25 of its 500 stores at first
- Debenhams will reopen 50 stores in England and three in Northern Ireland
- Marks and Spencer will open all its shops in England and Northern Ireland
- JD Sports is reopening all 309 of its stores in England
- Primark is reopening all 153 of its stores in England
- Argos, which has kept outlets in Sainsbury's stores open, will reopen 145 standalone stores for click and collect
- Topshop is reopening all its stores in England and selected ones in Northern Ireland
- Music retailer HMV is reopening 93 of its 113 stores
- Currys PC World is reopening 131 of its outlets.
Some other retailers selling products classed as essential - such as DIY, furniture and bicycles - have already started reopening. Furniture giant Ikea opened 19 of its stores across England and Northern Ireland recently, prompting long queues.
Many stores are encouraging customers to make purchases by contactless card payments, with limits increased to £45.
London's West End, which includes Oxford Street, is expecting about 80% fewer visitors when it reopens on Monday.
The British Retail Consortium (BRC), the trade body representing the sector, warned that the unlocking was unlikely to provide an immediate boost for the sector. It wants the government to help stimulate demand with a short-term cut in VAT or a temporary income tax cut for lower-income workers.
Zoos, safari parks and drive-in cinemas were also allowed to open from Monday.
In Northern Ireland, non-essential shops reopened on Friday, but there is still no date for Wales and Scotland.
In England, pubs, restaurants, hairdressers, hotels and cinemas will not be allowed to open their doors until 4 July at the earliest - and even then, only if they can meet social distancing measures.
Relaxing the 2m social distancing rule will be key to reviving the economy, the hospitality industry says. Pubs and restaurants have warned that they cannot make money if customers are required to keep two-metres apart, and have urged that the distance limit be reduced to one metre.
Prime minister Boris Johnson has commissioned a review into two-metre distancing, saying there was "margin for manoeuvre" as the number of coronavirus cases falls.
The government is under intense pressure to get the economy moving amid fears that there could be another wave of job losses as wage support schemes are wound down.
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