The festival, which ends on June 28, will feature a massive celebration and a film festival, Efe news agency reported.
This year it is also marked by a police ban on the main demonstration in defence of gay rights.
Authorities refused permission for the first time in 16 years to celebrate the parade on grounds that it could trigger clashes with counter-protesters, the majority of them being Protestant Christians.
The organisers said that they would go ahead with the parade despite the police ban.
Around a 100 radical Christians concentrated on Tuesday in Seoul City Hall, headquarters of the festival, with posters and sound equipment to boycott the opening ceremony over which the organisers fear the first frictions may occur.
In any case, the organisation expects a low attendance by the lesbian, gay, bisexual and transgender (LGBT) community and their supporters because of the MERS virus outbreak in the country which has spread fear among the people from going to crowded places.
Tensions arose this week after a message was spread on social networks by an anti-gay activist, accusing homosexuals of being a collective risk of transmitting the MERS since “they have HIV”.
This attack has sparked unrest and protests among the gay community, as reported by the The Korea Observer newspaper.
The Gay Pride Parade in Seoul was first held in the year 2000 with an approximate attendance of 50 people, and in 2014 it registered some 20,000 participants, according to the organisers.