Coronavirus tests for UK key workers were booked up through the government website within an hour of it reopening, apart from some in Scotland.
The site had to close to new applicants within hours of launching on Friday, after 46,000 people tried to access it. Some 16,000 bookings were made.
Home testing kits became unavailable less than 15 minutes after bookings reopened on Saturday morning.
Tests at drive-through sites in England were booked up within an hour.
Requests for drive-through tests in Scotland are still currently available on the site.
For key workers in England and Scotland with symptoms of the virus, No 10 has said appointments for tests at drive-through centres and home testing kits would become available each day from 08:00 BST, with their release staggered throughout the day.
Dr Simon Eccles, chief clinical information officer at NHS Digital, said an "amazing team" had worked "all night" to enable the site to reopen on Saturday.
After home testing kits were booked up, he wrote on Twitter: "I know it's frustrating but we're developing more lab, supply and logistics capacity every day.
"If we'd waited until we had the full 100k, to launch, no-one would have had a test today."
Prof Stephen Powis, medical director of NHS England, urged key workers showing Covid-19 symptoms to book a test online.
He told BBC Breakfast there was now capacity for more than 50,000 daily tests – and that reaching 100,000 a day by Thursday remained "the aim".
"The NHS has committed to capacity of 25,000 within NHS laboratories and we are on trajectory for that capacity to be in place," he said.
Prof Powis added the test people can book online is a swab test to determine whether people currently have the virus – rather than whether they have had it in the past.
Natalie's story: 'Disgusting waste of time and fuel'
Natalie Orton-Rose, from Leicester, said she managed to book a test on Friday but was turned away after she arrived at the drive-through centre.
The property manager for Poundstretcher, who is classed as a key worker, said she has been self-isolating after being told by her doctor that it was likely she had Covid-19.
Natalie, 34, said she requested a test at 07:00 BST on Friday and "finally" got a code to book an appointment at 15:00.
"I drove an hour from my home in Leicester [to the test centre in Nottingham] and sat waiting for half an hour in the queue only to be told actually they had no more tests left," she said.
"I am absolutely disgusted. It is bad enough that my closest test centre is an hour away but then to waste my time and fuel… I think the government and public need to be aware that just because you have an appointment and turn up doesn't mean you'll get the very much needed test."