About 70 countries throughout the world such as China and Iran have been known to use filters to cut their people off from the global web.

Foreign news media, websites of political opponents, and pages flagging up human rights abuses are routinely blocked.

Social media sites which allow groups to organize themselves online are also targeted and sometimes simply disconnected, as happened during the revolutions in Libya and Egypt.

But, as BBC Monitoring's Duncan Stanworth explains, there are ways to get around around online censorship.