Eleven organisations plan to stage march to oppose extradition to the US of former CIA employee behind NSA spying claims

Hong Kong activists are planning a rally in support of Edward Snowden, in an indication of the legal and social controversies that could unfold in the city as the NSA whistleblower's case unfolds.

Snowden, whose identity was revealed by the Guardian on Sunday, arrived in Hong Kong on 20 May and spent three weeks holed up in a luxury hotel before checking out on Monday afternoon. Snowden's location is unknown, but he is widely thought to still be in the city.

Hong Kong, a city of 7 million people, is technically a part of China but maintains an independent judiciary, media and education system. The territory has an extradition agreement with the US, but it can offer protection for fugitives who face political persecution or torture at home.

A group of 11 local organisations, many of them human rights NGOs, plans to hold the rally at 3pm on Saturday, according to a press release posted online on Wednesday afternoon. "We call on Hong Kong to respect international legal standards and procedures relating to the protection of Snowden; we condemn the US government for violating our rights and privacy; and we call on the US not to prosecute Snowden," it said.

Experts say that if the US does issue an extradition request, which it probably will do, Snowden could lodge a series of appeals, causing the resulting legal battle to stretch on for months or years.

Snowden told the Guardian he had fled to Hong Kong because of its "strong tradition of free speech". In doing so he galvanised local activists and politicians who claim that the territory's press freedom has eroded in recent years under pressure from Beijing.

"The environment is being changed very slowly, with self-censorship in media and so on," said Charles Mok, a member of Hong Kong's legislative council who will give a speech at the rally. "We definitely see this trend, with more and more people trying to behave and not upset Beijing.

"Making that comment: that's a good thing for us to hear, from somebody who is in the middle of a story like this."

The rally's press release said the participants would march past a Hong Kong government office and the city's US consulate, and suggested that they bring posters reading: "defend free speech, protect Snowden", "no extradition", "respect Hong Kong law", "shame on NSA", "stop internet surveillance" and "betray Snowden = betray freedom".

Tom Grundy, one of the protest's organisers, said in an interview: "Anyone who uses the internet and expects some privacy should be concerned about what was said in [Snowden's] interview, so I imagine we will get a good turnout."