Parliament's home affairs committee is questioning Rusbridger as part of its investigation into counter- terrorism, amid claims the newspaper endangered national security by publishing details of US and British spying.
Britain's top spy chiefs warned last month that Al-Qaeda and other enemies were "lapping up" Snowden's revelations and were using them to change the way they operate.
The Guardian counters that its stories sparked an important debate about intelligence, privacy and freedom of speech. It insists it has handled all the information sensitively.
Ahead of the parliamentary hearing, Rusbridger, 59, tweeted a "v nice letter" of support from Carl Bernstein, the veteran US journalist who helped break the Watergate scandal.
Bernstein said the hearing appeared to be "an attempt by the highest UK authorities to shift the issue from government policies and excessive government secrecy in the United States and Great Britain to the conduct of the press".
"Rather than hauling in journalists for questioning and trying to intimidate them, the (House of) Commons would do well to encourage and join that debate," he added.
The Index on Censorship campaign group raised similar concerns in an open letter to the chairman of the home affairs committee, opposition Labour lawmaker Keith Vaz.
"We are concerned that rather than a debate being opened up, the focus has instead been on criticising the Guardian's work, with even the prime minister threatening to take action against the newspaper if it did not take 'social responsibility'," Index chief executive Kirsty Hughes wrote.