Privacy campaigners held the second national day of protest against US government surveillance programmes on Sunday, picketing an AT&T building in New York they claimed was being used to facilitate National Security Agency snooping.

A grassroots group named Restore the Fourth, after protections against unreasonable search enshrined in the constitution, staged a series of events in several US cities to draw attention to the domestic spying programmes revealed by NSA whistleblower Edward Snowden.

Organisers said they felt the political mood had shifted in their favour since similar demonstrations were staged on 4 July, shortly after the Guardian first broke news of Snowden's revelations.

"The tide has turned against the National Security Agency's unconstitutional phone and internet monitoring," said Ben Doernberg from Restore the Fourth NYC. "The front page of Monday's New York Times stated that opposition to the NSA's activities has 'momentum that even critics say may be unstoppable,'"

Speaking shortly before the march arrived at the AT&T building in midtown Manhattan targeted by protestors, Doernberg estimated around 400 people had joined the New York rally.

A similar event in San Francisco was targeted at House minority leader Nancy Pelosi, who is criticised by the group for pressuring Democrats to oppose the Amash amendment, a narrowly defeated measure to rein in the NSA.

Pentagon Papers whistleblower Daniel Ellsberg was due to speak in San Francisco, while speakers from the ACLU of Massachusetts, Digital Fourth and other organisations were planning to speak at a "Bricnic" at the Boston Regional Intelligence Center (Bric).

The 4 August events were dubbed "1984 day" after George Orwell's dystopian warnings of all-powerful government surveillance.

"1984 is a warning, not an instruction manual," said Andrea O'Neill of Restore the Fourth DC. "As we find out about more unconstitutional programs every week, it is clear that the NSA's domestic spying has gone too far and must be stopped before it's too late."

Campaigners plan to spend the next few weeks targeting politicians in their local communities during the summer Congressional recess, before returning with more national events in October.