Ubaydullah Hussain, who was convicted of hate speech against Jews in February, launched the site on Wednesday with an attention-grabbing image of a rainbow flag in flames.
"This group is created to show our opposition to "gay pride (which is really gay shame)," Hussain wrote in his first post on the site, stressing that homosexuality is "strongly forbidden" under Islam, backing up his claim with quotations from the Qur'an.
The page, titled "Stopp homofile i Oslo`s gater!" or "Stop homosexuals on Oslo's streets," had 287 members by Thursday afternoon.
Yousef Assidiq, a Norwegian who converted to Islam in 2009 and campaigns against Islamophobia and Islamic extremist, said he was "disappointed" by Hussain's site.
"I believe in tolerance and I hope that Muslims can get together and support the pride festival on Saturday, and at least ensure that there's no violence or hatred coming out that day," he told The Local.
Assidiq earlier posted a call to Muslims to defend the rights of gay people.
"We Muslims expect to be able to live our daily lives without being harassed, threatened or attacked. We require to be defended from attack, intolerance and hatred," he wrote. "Then we should make sure that all the other people get the same security and the same rights."
Ubaydullah Hussain was formerly the spokesman of Profeten's Ummah, a controversial extremist group which has praised Osama bin Ladin and also claimed to be involved in recruiting Norwegians to fight in Syria.
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Hussain's involvement in the group brought an end to a promising career as a referee, after he was banned by the Norwegian football association because of his extreme views.
Stein Runar Østigård, the chairman of Oslo Pride, said Islamic opposition to the festival had "almost become a tradition".
"This is just sad," he added. "I hardly know what to say."