The European Union says Serbia's decision to ban a gay pride parade is regrettable, and "a missed opportunity to show respect for fundamental rights".
Serbian police have been deployed in the capital Belgrade to enforce a ban on a gay pride parade for the third year running, an act the EU says could undermine Serbia's chances of joining the bloc.
Hundreds of police in full anti-riot gear surrounded a park in downtown Belgrade on Saturday where the parade was due to take place, as well as main buildings and key crossroads.
On Friday Serbian Prime Minister Ivica Dacic announced the parade would have to be cancelled after far-right groups threatened protests.
The European Union said it expected Serbia to probe the threats of violence and "take necessary measures" to prevent them in future.
But it said the decision to ban the parade was regrettable.
"It is a missed opportunity to show respect for fundamental rights," EU Enlargement Commissioner Stefan Fuele said in a statement.
"I am committed to seek from candidate countries that they fully embrace values such as freedom of assembly and freedom of expression that are amongst the core foundations on which the European Union project is built.
"This is particularly important in the context of Serbia's accession negotiations."
Serbia, a candidate for EU membership since mid-2012, is expected to start accession talks with Brussels in January, but human rights activists have already warned it could be hampered by the ban of the gay pride parade.
The organisers said they would lodge an appeal against the ban.
The last gay pride parade in Serbia in 2010 ended in violence, with more than 150 people injured, mostly police, and the damage estimated was more than one million euros ($A1.45 million).