One of the organisers, Nikolai Alexeyev, said early Wednesday that he would fight a ban in court.
"We have prepared answers to all requests submitted by organisers and we will not accept them," Interfax news agency quoted city hall official Alexei Mayorov as saying.
Mayorov said the ban was motivated by principles of "morality ... and patriotism" that had to be taught to younger people.
A gay rights group had asked for authorisation of a May 25-26 demonstration and march in a Moscow park on Monday.
Human rights activist Lyudmila Alexeyeva, a founding member of the Moscow Helsinki Watch Group, denounced the ban,which has been enforced for years, as homophobic.
"In our country homophobia is state-sponsored," Interfax news agency quoted her as saying.
Clashes between gay rights activists who try to circumvent a ban and religious groups are recurrent in Russia where homosexuality was a crime until 1993 and a mental condition until 1999, almost a decade after the fall of the Soviet Union.
Saint Petersburg, Russia's former imperial capital, last year adopted legislation that punishes publicly "promoting" homosexuality and paedophilia, a link rejected by rights activists.