However, the court said the sharing of information with foreign government was not in breach of the rules.
"While some activists may expect the court to outlaw all bulk interception, the court seems to say the principle is okay - it's the government's practice which was illegal".
The landmark judgment marks the court's first ruling on United Kingdom mass surveillance programmes including bulk interception of communications, intelligence-sharing with foreign governments and obtaining of data from service providers.
The first two were found to breach the convention, while the latter did not.
But it said such programmes require sufficient oversight to keep the surveillance to what is "necessary in a democratic society".
It said it was not persuaded that the collection of this information was less intrusive than the acquisition of content, pointing out that content might be encrypted or, if decrypted, might reveal nothing of note.
They wrote: "In view of the potential chilling effect that any perceived interference with the confidentiality of journalists' communications and, in particular, their sources might have on the freedom of the press, the Court found that the bulk interception regime was also in violation of article 10".
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In contrast, related communications data is capable "of painting an intimate picture of a person" through mapping social networks, location tracking and insight of who they interacted with.
In a further victory for the 16 complainants the court ruled that the programme also provided "insufficient safeguards in respect of confidential journalistic material", violating Article 10 of the European convention, which protects freedom of expression and information.
The suit was brought by Big Brother Watch, Amnesty International, the American Civil Liberties Union, and a number of other civil liberties organizations from Europe and North America, as well as the Bureau of Investigative Journalism and others. The court said it is "satisfied" that British intelligence services take their human rights convention obligations seriously "and are not abusing their powers".
The Court's ruling isn't yet final: A Chamber judgement can be referred to the Grand Chamber any time within three months of its issuance, upon which a panel of five judges will decide whether the case merits further examination: If it does, it will be re-heard for a final judgement; if not, the Chamber judgement becomes final.
The case centred on powers given to security services under the Regulation of Investigatory Powers Act 2000 (Ripa), which has since been replaced. The High Court, whose ruling applied to Part 4 of the Act, gave the government until 1 November to change the law.
'This landmark judgment confirming that the UK's mass spying breached fundamental rights vindicates Mr Snowden's courageous whistleblowing and the tireless work of Big Brother Watch and others in our pursuit for justice, ' claims Big Brother Watch director Silkie Carlo. "This judgement is a vital step towards protecting millions of law-abiding citizens from unjustified intrusion".
But that argument doesn't convince critics of the Investigatory Powers Act - legislation which those opposed to it have previous labelled as "the most extreme surveillance law ever passed in a democracy".
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