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Twitter co-founder defends app and talks about new project

21 November 2014 Last updated at 18:18 GMT

The chairman and co-founder of Twitter, Jack Dorsey, has been in London for the global launch of his new company 'Square'.

The free app turns a tablet or smartphone into a sales register that can be used to process payments.

He spoke to the BBC's Rory Cellan-Jones and defended Twitter against criticism including its use by terrorists and its power to end politicians' careers.