More than 200 million US mobile phones have received a test "Presidential Alert" notification.
The trial is designed to check that a previously unused emergency communications system works properly.
Unlike other alerts - such as natural disaster warnings - there was no way to opt out, except switching a device off or otherwise blocking its connection.
Some have described the test as a "Trump Alert" - but the US leader was not personally involved in the trial.
Instead, the nationwide event is being run by the Federal Emergency Management Agency (Fema), which would also be in direct control of the system if a real alert was ordered by the US President, Donald Trump.
It is intended to be used to warn of major threats, including:
The alert produced a tone and showed a notification saying: "THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed."
It was being broadcast from mobile phone masts for about 30 minutes from 14:18 ET (19:18 BST) - with devices displaying only a single message.
The test was mandated under a 2015 law that said one must be run at least once every three years.
It was originally scheduled for September but was delayed in order to avoid any confusion while Hurricane Florence response efforts were continuing in North and South Carolina.
After the alert, Twitter and Facebook immediately lit up with people discussing the merits of the system, or complaining that they had not received the notification.
Others posted parody alerts, featuring photoshopped images.
Congress has placed limits on when the president can trigger such a warning, saying it must relate to a natural or man-made disaster or public safety threat.
Even so, there was a last-minute effort to block the test and wider use of the Presidential Alert system.
People look at their phones during the alert in New York's Grand Central Station
A journalist, a breastfeeding advocate and a fitness instructor teamed up to sue Fema, claiming the technology was a violation of their rights to be free from "government-compelled listening".
The legal action alleged that the test could be "traumatic" to children and that the system was open to abuse.
"Officials - including [President] Trump - are free to define 'act of terrorism' and 'threat to public safety' as they see fit, potentially broadcasting arbitrary, biased, irrational and/or content-based messages to hundreds of millions of people," the legal action claimed.
However, a New York judge refused the request at a hearing on Wednesday morning.
Others have used social media to complain about and mock the trial. Several have noted that anyone wanting to know the president's thoughts can turn to Twitter.
But in response, others have highlighted that the system was developed and put in place during the presidencies of George W Bush and Barack Obama.
Moreover, after two previous alert system errors - the first involving a false alarm about a missile attack on Hawaii, the second involving a presidential proclamation about Senator John McCain's death being sent to the wrong people - there is an argument that the technology needs to undergo regular public tests if it is to be relied on.
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