"We have to assume that, alongside the well-known edged and blunt weapons, firearms and explosives will be used," a memo to all the city's police stations said.
“All engaged forces are requested to particularly check vehicles from Denmark, Sweden, Norway and Finland,” it said, according to the BZ newspaper on Monday.
A police crackdown on Hells Angels and Bandidos - the biker gangs involved in organized crime across Germany - has thrown the scene into chaos, the paper said.
A new "Rocker Task Force" has been set up to make life as difficult for them as possible.
"Our line is zero tolerance," state prosecutor Jörg Raupach, who leads the task force, said on Saturday. "Whether it's abuse, driving without a license, drink driving - we will prosecute every crime. No investigation into the bikers - no matter how small - will be given up."
The Hells Angels "Berlin City" chapter was banned by state Interior Minister Frank Henkel in May, though that has done little to prevent the increasing violence.
Former Hells Angels leader André Sommer was gunned down in June, and several gang members subsequently appeared at the entrance to Berlin's Charité hospital to guard the 47-year-old as he lay in an artificial coma.
Two Bandidos were have also been shot at in the past few days.
Police believe that Berlin is home to between 400 and 500 Hells Angels, and slightly fewer Bandidos, and that the two groups are currently locked in a deadly power struggle over the city.
The gangs are thought to be involved in a number of organized criminal activities, including protection rackets, drug and weapons dealing, and human trafficking for prostitution.
But police have struggled to solve the crimes, because victims are reluctant to speak. "As long as no victim talks, we won't get much further," said Raupach. "We have to get a foot in the door."
Investigators are also hoping to help gang members get out of the scene. "We're trying to break one or two out," the prosecutor added. "And we have managed, but there are no quick successes."
"But our signal to the criminal scene is clear," he concluded. "We're watching you. We're always on your heels."
A former Hells Angels associate was jailed for more than four years in the northern city of Kiel last month. He was placed in a witness protection scheme after revealing details of regional biker gangs.
His testimony prompted huge raids across three states in May – and the destruction of a warehouse where police hope to find the remains of a missing man in the building's concrete foundations.