Human Rights Watch urged Singapore to drop new licencing rules for news websites, saying the "onerous" regulations would limit access to independent media.
The new rules, which require popular news websites to obtain an annual licence, will discourage independent reporting and curb free expression in online communities, the US-based watchdog said in a statement, released on Friday.
Volunteer-run blogs focusing on social and political issues including poverty and immigration have gained popularity as an alternative source of news and opinion in Singapore, where the mainstream media is widely seen as pro-government.
These blogs and other online social media sites have also been used by citizens to vent their frustrations and criticisms of government policies, often setting the tone for political debate in a country governed by strict laws against dissent.
"The Singaporean government should withdraw an onerous new licencing requirement for online news sites," Human Rights Watch said in a statement.
Cynthia Wong, the organisation's senior Internet researcher, added: "Singapore's new licencing requirement casts a chill over the city-state's robust and free-wheeling online communities, and will clearly limit Singaporeans' access to independent media."
She warned that the new rules could harm Singapore's reputation as a business centre.
"Singapore is placing its status as a world-class financial centre at clear risk by extending its record of draconian media censorship to the digital world."
Under the rules, which kicked in on June 1, websites with at least 50,000 unique visitors from Singapore every month that publish at least one local news article per week over a period of two months must obtain an annual licence.
Websites granted a licence will have to remove "prohibited content" such as articles that undermine "racial or religious harmony" within 24 hours of being notified by Singapore's media regulator, the Media Development Authority.
The rules have caused an uproar in the online community, which largely sees it as a measure to muzzle freedom of speech.
More than 130 Singaporean bloggers blacked out their homepages on Thursday and will hold a rally on Saturday to protest the new rules.
But Singapore's media regulator and other government officials have sought to allay fears that the new rules will impinge on Internet freedom.
Minister for Communications and Information Yaacob Ibrahim said Tuesday the government would continue to take a "light touch" approach to regulating the Internet and noted that blogs were not covered by the rules as they are not considered news portals.
Explore further: Huge YouTube bulk makes it tough to catch all terror videos