Jesselyn Radack, whistleblower and director of the Government Accountability Project, speaks during the Stop Watching Us Rally protesting surveillance by the U.S. National Security Agency, on October 26, 2013, in front of the U.S. Capitol building in Washington, D.C. Allison Shelley/AFP/Getty Images

Thousands of protesters marched Saturday on Capitol Hill in Washington to protest against the National Security Agency's spying programs.

Saturday’s protest comes amid a widening scandal revealing sweeping U.S. surveillance on the communications of ordinary citizens and global leaders that has sparked outrage worldwide.

German spy chiefs will travel to the U.S. next week to demand answers following allegations that the NSA has been tapping Chancellor Angela Merkel's mobile phone.

The U.S. may have bugged Merkel's phone for more than 10 years, according to a report in German magainze Der Spiegel.

The news report on Saturday also said that President Barack Obama told the German leader he would have stopped it happening had he known about it.

The reports of bugging prompted Germany to summon the U.S. ambassador this week for the first time, an unprecedented post-war diplomatic rift.

Der Spiegel said Merkel's mobile telephone had been listed by the NSA's Special Collection Service (SCS) since 2002 -- marked as "GE Chancellor Merkel" -- and was still on the list weeks before Obama visited Berlin in June.

The furore has intensified after allegations that world leaders including the presidents of Brazil and Mexico have been among spying targets.