Millions of US mobile phones have buzzed and beeped during the first test of a presidential alert system that would warn the public of a national emergency, such as an imminent attack.

The test message from the Federal Emergency Management Agency (FEMA) went out at 2.18pm, eastern daylight time (4.18am AEST), and triggered a loud tone, a vibration and a message reading: “THIS IS A TEST of the National Wireless Emergency Alert System. No action is needed.”

A separate alert on television and radios was issued two minutes later.

Officials estimated upwards of 225 million US mobile phones and other devices such as smart watches would receive the alerts, or about 75 per cent of all devices.

Some people reported not getting the alerts or getting delayed alerts. US mobile phone users are not able opt out of presidential alerts.

The test was the most discussed topic on Twitter, in part because of President Donald Trump’s propensity for sending tweets to his 55 million followers.

Officials told reporters on Tuesday that Trump would not personally trigger the alert and emphasised no president could “wake up one morning and attempt to send a personal message”.

As you can see, the alert system font was created with a word processor in 1998 pic.twitter.com/GkJzGavpTo

— Sara Kehaulani Goo (@sarakgoo) October 3, 2018

The hashtag #PresidentialAlert became the top US trending topic on Twitter, where most users complained about the alert system or used the opportunity to crack jokes.

“Oh my. A ‘Presidential Alert’ emergency. I thought that was the entire Trump presidency,” actor and activist George Takei tweeted.

Others made edits to the alert, changing the text in the FEMA-controlled message to “The Clintons are coming!” or “I resign! No action is needed”.

Earlier on Wednesday, a federal judge in New York City rejected a request by three New York residents to block the test.