Earth Security

Earth Security was a police force in operation on Earth by at least the year 2540. It was a global police force which tended to the security needs of the state headed by the civilian President of Earth. In practice, however, it may have been under the operational control of the military. (TV: Frontier in Space)

In the years preceding the Second Great Dalek Occupation, Earth Security still existed within the Earth Alliance. Alby Brook was a B-grade operative at the time of the Dalek invasion of Mutter's Spiral. His supervisor was Ernst Tanlee. (AUDIO: Invasion of the Daleks)

When they visited Gallius Ultima in the 54th century, Kala Tace mistook the Fifth Doctor, Adric, Nyssa and Tegan Jovanka for members of Earth Security. (AUDIO: The Star Men)

Behind the Scenes
According to the novelisation only, Earth Security was a somewhat sinister organisation, which denied its prisoners even basic civil rights. Captain Gardiner, for instance, expressed great misgivings about being anywhere close to an Earth Security Cell. While slightly ambiguous in the televised version, the novel makes it clearer that Earth Security is definitively at the service of the military. It explicitly points out that the Third Doctor and Jo end up in an Earth Security cell because of a military tribunal in which there was "no legal representation" and judgments of the court were "final and binding, against which there [was] no appeal".