The whistleblower Edward Snowden accused the chair of the Senate Intelligence Committee of double standards on Tuesday, pointing out that her outrage at evidence her staff were spied on by the CIA was not matched by concern about widespread surveillance of ordinary citizens.

Snowden, the former contractor whose disclosures to journalists revealed widespread surveillance by the National Security Agency, was responding to an explosive statement by Senator Dianne Feinstein about the CIA’s attempts to undermine a congressional investigation into interrogation and detention.

In a surprisingly combative statement on the Senate floor on Tuesday, Feinstein, who has been widely criticised by privacy experts for failing to hold the NSA to account, accused the CIA of conducting potentially unconstitutional and criminal searches on computers used by her staff.

The remarks put the Democratic senator on a collision course with the CIA’s director, John Brennan, who strongly denied “hacking” the committee’s computers. Feinstein described the controversy as “a defining moment for the oversight of our intelligence community”.

In a statement to NBC News, Snowden said: “It’s clear the CIA was trying to play ‘keep away’ with documents relevant to an investigation by their overseers in Congress, and that’s a serious constitutional concern.”

Snowden, who is in Russia on temporary asylum, added: “But it’s equally if not more concerning that we’re seeing another ‘Merkel effect,’ where an elected official does not care at all that the rights of millions of ordinary citizens are violated by our spies, but suddenly it’s a scandal when a politician finds out the same thing happens to them.”

Snowden’s comment referred to the German chancellor Angela Merkel, who was outraged to discover her phone calls has been monitored by the NSA. Germany has raised concerns over wider surveillance programs, but critics argue that Merkel should have gone further in condemning the NSA’s dragnet monitoring of European phone and internet communications.

Feinstein, who said she was making her statement “reluctantly”, confirmed recent reports that CIA officials had monitored computer networks used by Senate staff investigators. Going further than previously, she referred openly to recent attempts by the CIA to remove documents from the network detailing evidence of torture that would incriminate intelligence officers.

She also alleged that anonymous CIA officials were effectively conducting a smear campaign in the media to discredit and “intimidate” Senate staff by suggesting they had hacked into the agency’s computers to obtain a separate, critical internal report on the detention and interrogation program.

Feinstein’s dramatic speech on the Senate floor will reignite the debate over the adequacy of congressional oversight of the intelligence community, which was first sparked by Snowden’s NSA disclosures. Documents revealed by Snowden revealed the committee had been misled in public hearings by senior intelligence officials.

“How Congress responds [to the CIA controversy] and how this is resolved will show whether the Intelligence Committee can be effective in monitoring and investigating our nation’s intelligence activities, or whether our work can be thwarted by those we oversee,” Feinstein said.