Police have arrested four naked people protesting San Francisco's nudity ban on the steps of City Hall.

The arrests on Friday came as the city's prohibition on going out in the buff took effect.

About a dozen people in various states of undress took to the streets as part of the protest, though only the fully nude were taken into custody.

A federal judge ruled this week that nudity was not protected free speech and upheld San Francisco's ban on most displays of public nudity.

The Board of Supervisors voted 7-4 in December in favor of the ban.

Activists challenging the measure had argued that the ordinance was unfair because it grants exceptions for nudity at permitted public events such as the city's gay pride parade.