The UN Working Group on Arbitrary Detention (UNWGAD) announced on Friday that Assange has been "arbitrarily detained" by Sweden and Britain since his arrest in London in 2010, and he should be afforded the right to compensation, Xinhua reported.
Speaking from the balcony of the Ecuadorian Embassy in London, where he has been taking refuge, Assange said: "How sweet it is. This is a victory that cannot be denied."
"It is a victory of historical importance, not just for me, for my family, for my children, but for the independence of the UN system," he added.
Holding a copy of the UN ruling, he continued: "What right do the governments of the US, UK or Sweden have to deny my children their father for five and a half years?"
However, British Foreign Secretary Philip Hammond described the ruling as "a ridiculous finding" by the working group, and accused Assange of "hiding from justice."
"Assange is a fugitive from justice, voluntarily hiding in the Ecuadorian embassy. I reject the report from UNWGAD," Hammond tweeted.
Hitting back at Hammond, Assange accused the foreign secretary' s comments of "insulting the UN."
"I found those comments to be beneath the stature that a foreign secretary should express in this situation," The 44-year-old Australian said.
Edward Snowden, a US intelligence contractor-turned-whistleblower who has fled the US and taken refugee in Russia, criticised Britain's rejection to abide by the UN ruling.
"This writes a pass for every dictatorship to reject UN rulings. Dangerous precedent for UK and Sweden to set," he said on his twitter account.
British Foreign and Commonwealth Office (FCO) denied that Assange has been arbitrarily detained by Britain, and threatened to formally contest the opinion of the UN Working Group.
"This changes nothing. We completely reject any claim that Julian Assange is a victim of arbitrary detention. The UK has already made clear to the UN that we will formally contest the working group's opinion," an FCO spokesperson said.
The spokesperson noted that an allegation of rape against Assange is still outstanding with a European Arrest Warrant in place, and Britain "continues to have a legal obligation to extradite him to Sweden."
Assange has infuriated several governments, including the US authorities, by disclosing hundreds of thousands of secret files concerning Iraq, Afghanistan and diplomatic cables from US embassies around the world.
In June, 2012, Assange fled to the Ecuadorian Embassy in London and was granted political asylum by Ecuador two months later.