Lawyers said the NSA whistleblower could make a credible case for rejecting a US application for his return, on the grounds that his alleged offence was political, but queried his prospects of ultimate success.

If the US decides to bring criminal charges and requests his surrender – technically not extradition, because Hong Kong is not a sovereign state – he might be able to argue that he should be exempt under the treaty's political exception provisions.

There is no set definition of a political offence: it is at the discretion of the state that receives the request. Both the intent of the authority making the request and the motivation of the alleged offender are considered.

Tim Parker, an immigration lawyer in the territory, said the chief executive of Hong Kong – which is part of China but enjoys considerable autonomy under the "one country, two systems" framework – would decide on the US request. But Snowden would then be able to challenge the decision through the courts.

"In controversial political cases it would likely go to the court of appeal and ultimately the court of final appeal," he said.

The process "could go on for months and easily into years".

Hong Kong has not accepted a political defence against extradition since the handover in 1997. In the 1930s it turned down a bid by French authorities for Ho Chi Minh's return to what was then IndoChina, in a case that went all the way to the privy council. More recently, in 1994, its courts stayed the extradition of the politician Jeffrey Kitigan to Malaysia.

Guy Goodwin-Gill QC, a leading expert on extradition at Blackstone Chambers in London, noted that the international climate had changed, with offences that previously might have qualified – such as hijacking – no longer accepted as political.

"The times are very much one of co-operation across a broad range of activities. You hardly ever find a state refusing extradition for political offences any longer," he said.

In the case of Snowden, "you certainly see political dimensions: you have members of the US senate or House of Representatives calling him a traitor – so they are building a very good case for another state to treat this as political," added Goodwin-Gill.

"Academically, you can certainly build a case for a political offence. Whether it would be sufficient to persuade the requested state is another matter."

Another international expert on extradition said courts have in general considered political offences to be those committed as an incidental part of a broader political power struggle between a defined movement or party of which the defendant is a member, and the established government.

Snowden might be also able to fight his surrender on the grounds he would face cruel, inhuman or degrading treatment or punishment if he were returned.

But if the US provided assurances that he would not be treated in such a manner, it might be hard for Hong Kong to resist, Patricia Ho, a Hong Kong lawyer who specialises in asylum and refugee claims, told Associated Press.

The UN special rapporteur on torture has formally accused the US government of cruel, inhuman and degrading treatment towards Bradley Manning, the US soldier currently on trial for leaking classified material to WikiLeaks. But the US might well argue that Snowden's treatment in the civilian system could not be compared to military detention.

Simon Young, the director of the centre for comparative and public law at the University of Hong Kong, noted that approval for surrender can only be made if there is an assurance that the death penalty will not be used.

Young added that a request would also be trumped by asylum proceedings. If Snowden applied for resettlement to a third country - Hong Kong does not grant asylum itself - that issue would have to be settled first. A legal ruling this spring has left the territory without a mechanism for handling such claims; it could take months for a system to be introduced.

Technically, the fugitive offenders ordinance would allow Beijing to step in after Hong Kong had decided whether to surrender Snowden. But it has not used these powers in the past and to intervene publicly would cause a storm.

Diplomatic discussions behind the scenes can prove as crucial as what happens in courts. Goodwin-Gill noted that a state may not ask for a suspect's extradition if it thinks its request will be rejected.

In the long run, suggested some experts, Snowden might stand a better chance of avoiding proceedings by going to a country that does not have an extradition agreement with the US. With no criminal proceedings yet under way there is, in theory, nothing to stop him simply boarding a flight to a new destination. But given his views on constraining state power and controlling surveillance, nearby options such as China, Vietnam or Laos would be bizarre choices – even if they let him in.

Jonathan Acton-Bond, a lawyer who has dealt with high-profile extradition cases in Hong Kong, told Reuters that Snowden might also face obstacles in leaving. "In strictly legal terms he's free to go, but government bodies can always find an excuse to temporise, or stop him," he said.