In 2007, the Supreme Court ordered the government to amend laws to end discrimination against homosexuals, and give them the same rights as other citizens.
Government officials were not immediately available for comment.
Kathmandu police chief Ramesh Kharel said the activists were detained for "violating the norms" by gathering at a place where demonstrations were not allowed.
Hindu-majority Nepal has become more gay-friendly over the last few years, but homosexuality still remains taboo for many people in this conservative Himalyan nation.
Same-sex marriages have taken place in public and gay beauty contests are held.
A travel agency run by gay people is offering to organise same-sex weddings at Mount Everest in a move to promote the scenic mountainous nation as a gay-friendly tourist destination.
(Reporting by Gopal Sharma; Editing by Sugita Katyal)
