The European Union's highest court ruled today that private individuals generally cannot install surveillance cameras to film people on a public path.
It said, however, that exceptions can be made if they help prevent and prosecute criminal acts.
The case arose after a Czech man installed a surveillance camera under the eaves of his family home from October 2007 to April 2008 after attacks, in some of which windows were smashed.
The camera filmed the entrance, public footpath and entrance of the house opposite. The man, Frantisek Rynes, handed over his recording to police and two suspects were prosecuted.
After one suspect complained, Rynes was fined by the Czech data protection authority for filming without consent on the footpath, thereby infringing the suspect's privacy.
Rynes appealed and his lawyers argued he was acting privately along the lines of a tourist filming a site with people captured in the footage.
The Luxembourg-based Court of Justice of the European Union (ECJ) said filming a public space from a private home cannot be considered a purely personal or household activity. But it said the Czech national court must take into account Rynes' legitimate interest in protecting the property, health and life of his family and himself.
The ruling comes amid debate in many countries on the increasing use of electronic surveillance by governments, businesses and other entities. While some say it bolsters security, others see it as an infringement on privacy and a form of social control.
"It clarifies privacy concerns with surveillance cameras," said Jan Albrecht of the Greens group in the European Parliament.
But one lawyer called it an "extremely surprising" decision, saying it could technically apply to people filming pigeons in London's busy Trafalgar Square.
"While the decision just looks at CCTV systems, it is likely to be relevant to a range of other technologies such as social media, smartphones and wearable technology," said Richard Cumbley, a partner at Linklaters.
The EU is currently negotiating a reform of its 20-year-old data protection laws to give individuals more say in how their personal data is collected and used.
The court said processing data without consent, in this case filming public paths, could be allowed if necessary to safeguard the prevention, investigation, detection and prosecution of criminal offences.
The judgement is not the final decision in the Czech case but the Czech court will have to take into account its arguments.