One of the UK's top cops has warned the battle against extremism could push the country towards a police state.

Greater Manchester Chief Constable Sir Peter Fahy said police were being put in the position of having to decide was what acceptable free speech, the Guardian has reported.

He warned the government, academics and activists needed to work together to establish the line between free speech and extremism.

Mr Fahy has experience on the issue, being the vice-chairman of the police terrorism committee and national lead on Prevent, the UK's counter-radicalisation strategy.

"There is a danger of us being turned into a thought police," he said.

However, he said he supported the UK government's recent raft of anti-terror laws and said academic institutions needed to do more work to identify and counter extremist speakers.

Mr Fahy also emphasised that he was not simply talking about Muslim extremism.