European governments' ability to monitor citizens' online activity on security grounds has increased significantly in recent years.
But the scale of surveillance remains limited compared to what has been happening in the United States and is generally subject to strict controls, officials say.
Under a directive approved by the 27-member European Union in 2006, telecommunications operators and internet service providers are obliged to retain records of users' calls and online activity for at least six months.
The directive enables security services, if necessary, to check who has been communicating with whom, from where, at what time and for how long but stops short of enabling them to check the content of any communications.
The European legislation, approved in the aftermath of bombings in Madrid in 2004 and London in 2005, is intended to establish a minimum standard across the bloc.
But individual governments are free to require longer periods of data retention by telecoms operators, some of whom have complained loudly about the additional costs involved.
In Italy, for example, the requirement has been four years for some time while the likes of Germany and Finland had no legislation in this area prior to the EU directive being adopted.
The underlying principle of the directive is that governments can only request the information on the grounds of reasonable suspicion of activity linked to either terrorism or organised crime.
The actual process involved in authorising checks on the data covered by the EU directive and whether intelligence service can go beyond it to check the content of emails etc., depends on each government.
In Britain, police can identify who has made a telephone call or sent a text message, and when and where.
However they cannot do the same for email, Internet, instant messaging or other Internet-based services such as Skype, and the police and security services say they need the power to do so in order to combat terrorism effectively.
Prime Minister David Cameron's Conservative party agree but draft legislation on the issue has been blocked by their coalition partners, the Liberal Democrats, amid fears it would become a "snoopers' charter".
Although the bill would have given police access to all online communication in Britain, they would still have had to seek a warrant from a government minister to check the content of conversations and messages.
Spain is moving in a similar direction with the government currently working on reforms to the penal code which will allow police to install spy software on individuals computers on the authorisation of a judge.
The reform is only at the proposal stage at the moment but, to date, the issue has not caused much controversy.
In Germany, security services are able to check the nature and content of all digital communications under legislation dating from 2002 and updated in 2008.
As in Spain, officers have to go to court and convince a judge that the security concern involved is sufficiently important for the right to privacy of the individuals targeted to be overridden.
In France, an examining magistrate in charge of a criminal case can require internet service providers to hand over records of communications they are required to keep for a year.
Legislation also provides for access to certain sites to be blocked and for spyware to be installed on computers to keep tabs on individuals suspected of involvement in terrorism, but also paedophilia, murder, torture, arms trafficking or the drugs trade.
Access to the content of messages is only permitted, with judicial approval, in exceptional circumstances within the framework of investigations concerning national security, terrorism or organised crime.
Explore further: Digital dilemma: How will US respond to Sony hack?