Edward Snowden, a former contractor for the CIA, left the US in late May after leaking to the media details of extensive internet and phone surveillance by American intelligence. Mr Snowden, who has been granted temporary asylum in Russia, faces espionage charges over his actions.
As the scandal widens, BBC News looks at the leaks that brought US spying activities to light.
US spy agency 'collects phone records'
The scandal broke in early June 2013 when the Guardian newspaper reported that the US National Security Agency (NSA) was collecting the telephone records of tens of millions of Americans.
The paper published the secret court order directing telecommunications company Verizon to hand over all its telephone data to the NSA on an "ongoing daily basis".
That report was followed by revelations in both the Washington Post and Guardian that the NSA tapped directly into the servers of nine internet firms, including Facebook, Google, Microsoft and Yahoo, to track online communication in a surveillance programme known as Prism.
Britain's electronic eavesdropping agency GCHQ was also accused of gathering information on the online companies via Prism.
Shortly afterwards, the Guardian revealed that ex-CIA systems analyst Edward Snowden was behind the leaks about the US and UK surveillance programmes.
He has been charged in the US with theft of government property, unauthorised communication of national defence information and wilful communication of classified communications intelligence.
UK spy agency 'taps fibre-optic cables'
The GCHQ scandal widened on 21 June when the Guardian reported that the UK spy agency was tapping fibre-optic cables that carry global communications and sharing vast amounts of data with the NSA, its US counterpart.
The paper revealed it had obtained documents from Edward Snowden showing that the GCHQ operation, codenamed Tempora, had been running for 18 months.
GCHQ was able to boast a larger collection of data than the US, tapping into 200 fibre-optic cables to give it the ability to monitor up to 600 million communications every day, according to the report.
The information from internet and phone use was allegedly stored for up to 30 days to be sifted and analysed.
Although GCHQ did not break the law, the Guardian suggested that the existing legislation was being very broadly applied to allow such a large volume of data to be collected.
GCHQ and NSA eavesdropping on Italian phone calls and internet traffic was reported by the Italian weekly L'Espresso on 24 October. The revelations were sourced to Edward Snowden.
It is alleged that three undersea cables with terminals in Italy were targeted. Italian Prime Minister Enrico Letta called the allegations "inconceivable and unacceptable" and said he wanted to establish the truth.
US 'hacks China networks'
After fleeing to Hong Kong, Edward Snowden told the South China Morning Post that the NSA had led more than 61,000 hacking operations worldwide, including many in Hong Kong and mainland China.
He said targets in Hong Kong included the Chinese University, public officials and businesses.
"We hack network backbones - like huge internet routers, basically - that give us access to the communications of hundreds of thousands of computers without having to hack every single one," Mr Snowden was quoted as saying.
EU offices 'bugged'
Claims emerged on 29 June that the NSA had also spied on European Union offices in the US and Europe, according to Germany's Der Spiegel magazine.
The magazine said it had seen leaked NSA documents showing that the US had spied on EU internal computer networks in Washington and at the 27-member bloc's UN office in New York.
The paper added that it had been shown the "top secret" files by Edward Snowden.
One document dated September 2010 explicitly named the EU representation at the UN as a "location target", Der Spiegel wrote.
The files allegedly suggested that the NSA had also conducted an electronic eavesdropping operation in a building in Brussels, where the EU Council of Ministers and the European Council were located.
It is not known what information US spies might have obtained. But observers say details of European positions on trade and military matters could be useful to those involved in US-EU negotiations.
Merkel phone calls 'intercepted'
The German government summoned the US ambassador on 24 October - a very unusual step - after German media reported that the NSA had eavesdropped on Chancellor Angela Merkel's mobile phone.
The allegations dominated an EU summit, with Mrs Merkel demanding a full explanation and warning that trust between allies could be undermined. She discussed the matter by phone with US President Barack Obama. He assured her that her calls were not being monitored now and that it would not happen in future. But the White House did not deny bugging her phone in the past.
Past surveillance by secret police - whether Nazi or communist - has made Germans very sensitive about privacy issues. Mrs Merkel grew up in the former East Germany, where the Stasi spied on millions of citizens.
France's President Francois Hollande meanwhile expressed alarm at reports that millions of French calls had been monitored by the US.
The Guardian later reported that the NSA had monitored the phones of 35 world leaders after being given their numbers by another US government official. Again, Edward Snowden was the source of the report.
Embassies 'under surveillance'
A total of 38 embassies and missions have been the "targets" of US spying operations, according to a secret file leaked to the Guardian.
Countries targeted included France, Italy and Greece, as well as America's non-European allies such as Japan, South Korea and India, the paper reported on 1 July.
EU embassies and missions in New York and Washington were also said to be under surveillance.
The file allegedly detailed "an extraordinary range" of spying methods used to intercept messages, including bugs, specialised antennae and wire taps.
The Guardian report also mentioned codenames of alleged operations against the French and Greek missions to the UN, as well as the Italian embassy in Washington.
US Secretary of State John Kerry said that activities to protect national security were "not unusual" in international relations.
Latin America 'monitored'
US allies in Latin America were angered by revelations in Brazil's O Globo newspaper on 10 July that the NSA ran a continent-wide surveillance programme.
The paper cited leaked documents showing that, at least until 2002, the NSA ran the operation from a base in Brasilia, seizing web traffic and details of phone calls from around the region.
US agents apparently joined forces with Brazilian telecoms firms to snoop on oil and energy firms, foreign visitors to Brazil, and major players in Mexico's drug wars.
Mexico, Brazil, Colombia and Chile all demanded answers from the US.
But the revelations on Latin America kept coming, and in September more specific claims emerged that emails and phone calls of the presidents of Mexico and Brazil had been intercepted.
Also, the US had been spying on Brazil's state-owned oil firm Petrobras.
Brazilian President Dilma Rousseff cancelled a state visit to the US in the most high-profile diplomatic move since the scandal hit.
US spying 'errors'
Documents leaked to the Washington Post in mid-August suggested the NSA breaks US privacy laws hundreds of times every year.
The papers revealed that US citizens were inadvertently snooped on for reasons including typing mistakes and errors in the system,
In one instance in 2008, a "large number" of calls placed from Washington DC were intercepted after an error in a computer program entered "202" - the telephone area code for Washington DC - into a data query instead of "20", the country code for Egypt.
Later in August, the Washington Post reported that US spy agencies had a "black budget" for secret operations of almost $53bn in 2013.
SMS messages 'collected and stored'
In January 2014, the Guardian newspaper and Channel 4 News reported that the US had collected and stored almost 200 million text messages per day across the globe.
A National Security Agency (NSA) program is said to have extracted and stored data from the SMS messages to gather location information, contacts and financial data.
The documents also revealed that GCHQ had used the NSA database to search for information on people in the UK.
The programme, Dishfire, analyses SMS messages to extract information including contacts from missed call alerts, location from roaming and travel alerts, financial information from bank alerts and payments and names from electronic business cards, according to the report.
Through the vast database, which was in use at least as late as 2012, the NSA gained information on those who were not specifically targeted or under suspicion, the report says.
The revelations came on the eve of an expected announcement by President Obama of a response to recommendations by a US panel on ways to change US electronic surveillance programmes.