The UK conducted its first test of a new emergency alert service on Sunday, with millions of mobile phones emitting a loud alarm and vibrating.
The national system, modelled on similar schemes in Canada, Japan, the Netherlands and the United States, aims to warn the public if there is a danger to life nearby but has generated criticism over "nanny state" intrusion.
The alert was due to go off at 3pm (1400 GMT), although some phones sounded the alarm before the scheduled time, and others minutes later.
Some users on social media complained that they had not received the warning at all.
The alarm was accompanied by a message reading: "This is a test of Emergency Alerts, a new UK government service that will warn you if there's a life-threatening emergency nearby."
Emergency services and the government hope to use the system to alert people to issues such as severe flooding and fires.
The 10-second alarm, which sounded even if phones were on silent, rang out at entertainment and sporting events, including Premier League football matches.
Organisers of the World Snooker Championship paused play just before the alert, while the Society of London Theatre advised its members to tell audiences to turn off their phones.
Drivers were warned not to pick up their phones during the test, and people who did not wish to receive the alerts were able to opt out in their device settings.
"Keep Calm and Carry On. That is the British way and it is exactly what the country will do when they receive this test alert at 3pm today," said Deputy Prime Minister Oliver Dowden before the test.
"The government's number-one job is to keep people safe and this is another tool in the toolkit for emergency situations."
'Terrifying'
But some Conservative figures have criticised the plan, with former minister Jacob Rees-Mogg urging people to defy the government's calls and "switch off the unnecessary and intrusive alert".
"It is back to the nanny state -– warning us, telling us, mollycoddling us when instead they should just let people get on with their lives," he said.
Daily Mail columnist Sarah Vine, ex-wife of government minister Michael Gove, called the plans "terrifying."
"This Sunday, at 3 pm... the government intends to rattle our collective cages by invading our mobile phones -- and our privacy -- with its absurd emergency test signal. The notion is as terrifying as it is tiresome," she wrote.
"Terrifying because it's a reminder of the tyranny imposed on all of us by the technology that has invaded our homes like Japanese knotweed, infiltrating every aspect of our daily lives," she added.
Dowden sought to play down privacy and intrusion fears, saying "all people need to do is swipe away the message or click "OK."
"The test is secure, free to receive and one-way, and does not reveal anyone's location or collect personal data," he added.
Judy Edworthy, an international expert in alarm systems and psychology professor at the University of Plymouth, said the alert system was a positive development, even if its first airing may surprise people.
"Despite the message explaining it is a test, I expect some people may well be astonished," she told the domestic Press Association.
MPs also criticised the decision to hand the lucrative IT contract for the alert system to Fujitsu, the Japanese firm responsible for faulty software in the Post Office system that led to innocent sub-postmasters receiving fraud convictions.
