Special police plates were experimentallly introduced in Moscow in 2001. The experiment seemed to be successful, as in a year "blue" plates appeared in other regions, too.

Police plates may be issued to cars and trucks, motorcycles and trailers. The vehicle plate is of a "one letter-four digits" format. A regional code reference the city of the vehicle registration. As an exception, code "77" refers to the Ministry of Internal Affairs (a kind of a "federal" police plate).

The letter is the key: it specifies the division or department where the vehicle belongs to. Below are the codes of the police vehicles.
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Kostroma Oblast (44)

À0001 44-0020 44 - Management of the Kostroma Oblast Police and the Federal Security Service offices
Â0100 44-0199 44 - District Departments of Internal Affairs
Â0200 44-0299 44 - Special Departments: Task Force, Economy Criminal Service, Organized Crime-Fighting Service
Â0300 44-0399 44 - Traffic Police

Leningrad Oblast (47)

Ð0100 47 - Ð9999 47 - District Departments of Internal Affairs

Moscow Oblast (50)


P****50 - Department of Internal Affairs
B****50 - Department of Non-Corporate Security
O****50 - Traffic Police, Moscow Oblast

Chelyabinsk Oblast (74)

A****74 - Central Department of Internal Affairs
B****74 - Non-Corporate Security
K****74, M****74 - Distric Departments of Internal Affairs
O****74 - Traffic Police
Y****74 - Logistic Service of the Central Department of Internal Affairs; Task Force

Ministry of Internal Affairs of the Russian Federation (77)

O****77 - Central Office, Traffic Police, General Security Service of the
Ministry of Internal Affairs
K****77 - Central Division, Organized Crime Fighting Service
M****77 - Sanitary and freight transport
Y****77 - Central Department, Non-Corporate Security

St. Petersburg City (78)


A****78 - Central Department of Internal Affairs
A0001 78 - Head of the City Central Department of Internal Affairs
A0002 78 - A0050 78 - Deputy Heads of the City CDIA
A0051 78 - A0060 78 - Department of Law Enforcement
A0061 78 - A0500 78 - Central Department of MIA
B****78 - Department of Non-Corporate Security
B0100 78 - B9999 78 - Non-Corporate Security
M****78 - Task Force
M 0100 78 - Task Force at the Central Department of Internal Affairs
O****78 - Traffic Police
O0095 78 - O9999 78 - Traffic Police
P****78 - District Police Departments
P0100 78 - P9999 78 - District Police Departments
C****78 - Regional District Police Department
T****78 - North-West Division of the Transport Police Dept.
Y****78 - Central Police Dept.; Central Division, MIA; MIA University; MIA educational institutions

Moscow City, (99)

A****99 - Central Dept. of Internal Affairs, Central Traffic Police Office
O****99 - Department of Non-Corporate Security
Y****99 - District Departments of Internal Affairs
P****99 - Regional Department, Organized Crime Fighting
K****99 - Regional Department, Organized Crime Fighting
T****99 - Traffic Police
Sources:
denissb.chat.ru
Sergey Teplygin
Please note: no official open source of the Russian Federation Ministry of Internal Affairs confirmed  this information, therefore it can deviate from the true state of affairs.
Konstantin Sevostyanov
E044KX44. Comment at the "Marked Power" Conference