PA/GETTY Micah Johnson shot dead five policeman in Dallas

FREE now and never miss the top politics stories again. SUBSCRIBE Invalid email Sign up fornow and never miss the top politics stories again. We will use your email address only for sending you newsletters. Please see our Privacy Notice for details of your data protection rights.

Officers in Baton Rouge, Louisiana, were yesterday on full alert after the interception of a chilling message that read in part: “Baton Rouge purge starts July 9 at 12am and ends at 5am July 10. Rule One: Must kill every police.” The warning comes after five policemen were shot and killed by sniper Micah Johnson in Dallas, Texas. The “purge” may be a reference to a hit film in which laws are suspended, allowing all crimes including murder to take place.

The FBI alert, leaked by watchdog Judicial Watch, was accompanied by disturbing images from social media, including one depicting a restrained uniformed officer having his throat slit by a masked assassin. Another message uncovered by the FBI urged protesters to join the purge by “killing all cops in Baton Rouge and starting a riot by the courthouse in Shreveport”. Within hours of the alert, the militant New Black Panther Party urged followers to join protests this weekend outside the city’s police headquarters, which it described as the “Baton Rouge Pig Department”.

GETTY A tribute to the fallen officers in Dallas

Dallas Shootings: Memorial Service Tue, July 12, 2016 Five Dallas police officers have been killed and seven wounded by sniper fire during protests against the shooting of black men by police. Play slideshow REUTERS 1 of 16 Former First Lady Laura Bush, former President George Bush, First Lady Michelle Obama and U.S. President Barack Obama hold their hands on their hearts as they sing the national anthem at a memorial service following the multiple police shootings in Dallas

Johnson’s gun rampage, which ended when the 25-year old Afghan war veteran was blown up by a robot bomb, was triggered by two police shootings last week that have enraged black activists and community leaders.

There is a growing underground movement that we are going to have to deal with Mark Fuhrman former detective

In the first, on Tuesday, unarmed Alton Sterling, 37, was shot dead in Baton Rouge after being pinned to the ground by police officers. The following day, 32-year-old Philando Castile was in a car with his girlfriend when he was shot by an officer in Minnesota. Officials now fear the killings, both of which are being investigated, have sparked an anti-police uprising that could sweep across the country. In the 48 hours since Johnson’s one-man murder spree in Dallas, police in Tennessee, Georgia and Missouri have all reported non-fatal attacks on officers.

GETTY The FBI intercepted a message about a 'purge' in Baton Rougue

Mark Fuhrman, a former Los Angeles detective who is now a TV law and order commentator, said: “There is a growing underground movement that we are going to have to deal with. “Officers on the street must be especially alert to the possibility of being ambushed, possibly as the result of a fake emergency call. “All officers should carry ballistic helmets, vests and other protective gear and ensure they are always instantly to hand.”

REUTERS The badge of a Los Angeles Police Department sergeant is seen with a black band across it