Turkish authorities say they will not allow the Istanbul Pride march to take place on Sunday - the third year in a row the celebration has been banned.
The move prompted criticism from rights groups and fears of possible violence, as Pride organisers said they would defy the ban.
For more than a decade, Istanbul Pride has attracted tens of thousands of participants, making it one of largest gatherings celebrating gay, lesbian and transgender rights and diversity in the Muslim world.
Unlike other Muslim countries, homosexuality is not a crime in Turkey.
However, lesbian, gay and transgender activists say they lack legal protections and face widespread social stigma in a nation heavily influenced by conservative and religious values.
The Istanbul governor's office said the Pride march would be banned to keep public order and for the safety of participants and tourists.
It said the area around central Taksim Square, where the march begins, was not designated for demonstrations.
The volunteer Pride committee said the ban violates domestic and international law limiting the right to peaceful assembly.
It asked the governor's office to reconsider and fulfill its obligations by providing security precautions.
The city government also said "very serious reactions by different segments of society" were raised against the march.
This week, like last year, ultra-nationalist and conservative groups said they would not allow the Pride march to take place even if the authorities allowed it.
LGBTI activists said the ban legitimises threats and hate speech under the guise of protecting the public's "sensitivities".
Amnesty International expressed "deep worry" following the ban and said Turkish authorities violated freedom of expression and assembly in a "routine and arbitrary way".
"Turkey should protect rather than ban Pride marches," Amnesty said, adding that it would document developments on Sunday.
Up to 100,000 people took part in 2014's Pride march, making it one of the largest LGBTI Pride events in a predominantly Muslim nation.
The following year, authorities banned the march in a surprise move citing public order and dispersed the crowds.
In 2016, the march was again banned amid a spate of deadly attacks blamed on the Islamic State group or on outlawed Kurdish militants.
LGBTI activists still attempted to converge on Taksim Square, leading to skirmishes with police.
A state of emergency declared after last summer's failed coup has further limited public gatherings.
Organizers believe the celebrations in 2015 and 2016 were banned because they coincided with Islam's holy month of Ramadan.
They say authorities are using security as an excuse to ban the parades instead of taking measures to deal with the threats against those participating.
Sunday's planned march coincides with the Eid holiday, marking the end of a month of fasting for Ramadan.
"(The bans are) a reflection of the increasingly conservative and majoritarian policies of the government," said Murat Koylu, of the Ankara-based Kaos GL, a group promoting LGBTI rights.
"The fact that the existing political power is not making the necessary changes in the constitution, and the fact that they have discourse against us might encourage people who are already (trans) phobic," said Seyhan Arman, a 37-year-old transgender woman and performer.
The Turkish government insists there is no discrimination against individuals based on their sexual orientation, and that laws barring discrimination on the basis of gender, race, ethnicity or religion protect all citizens.
It also insists that perpetrators of hate crimes are prosecuted.
"The violence against us has existed since the day we were born. It starts in the family, it continues at the university, in the working life," said Deniz Sapka, a 27-year-old transgender woman originally from Hakkari, who goes by that surname to avoid recognition by family members.
"We are people who have always experienced a state of emergency. We experience it from our birth."