If you get an urgent message on your cellphone from President Obama later this year, it’s not a prank.

Under an emergency-notification system to be announced today by Mayor Bloomberg and federal officials, anyone carrying an “enabled” mobile device within range of a cellphone tower would be alerted about what to do in case of emergency.

“If there’s a terrorist bomb set to off go off in Times Square, you’d be able to tell everyone in the vicinity to get out of there,” explained one official. The service is scheduled to be available in New York City and Washington by the end of the year and in the rest of the country by mid-2012.

But to receive the alerts, cellphones and other wireless devices will need a special chip that is now only in a few models of mobile phones. Cellphone companies have agreed to add the chip to new phones and wireless devices rolling out by 2012.

Officials said the alarms would fall into three categories: a warning issued directly by the president, a message involving imminent threats to safety, and Amber Alerts about missing kids. New York City already has a method to contact residents in a hurry via cellphone, but only for those who sign up in advance. Since its launch in May 2009, Notify NYC has attracted 67,000 subscribers and 17,000 Twitter followers.

The announcement will be made today at the World Trade Center site.