An Islamist extremist group leader told a university crowd ex-Muslims should be killed under Sharia law during a debate with an atheist who feared for his safety.

Hizb ut-Tahrir spokesman Uthman Badar clarified his position on apostates who leave Islam at the University of Sydney.

His atheist opponent Dr John Perkins, who is also Secular Party of Australia president, diverted from the debate topic about the existence of God to ask Mr Badar about his position on killing ex-Muslims.

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Hizb ut-Tahrir spokesman Uthman Badar says ex-Muslims should be killed in an Islamic state

'I would like to again ask Uthman about the killing of apostates and whether he does support it or whether he does renounce it?,' Dr Perkins said.

He had told Daily Mail Australia before the debate he feared for his safety and had needed security during a previous debate with Mr Badar.

Rather than renounce his support for killing ex-Muslims, Mr Badar used weasel words at the University of Sydney debate to confirm his view they should be given the death penalty where Sharia law applied.

'The idea that I have called for ex-Muslims in Australia to be killed is a fabrication of tabloid media,' he said.

'As a matter of principle or a penal code that is part of Islam, I'm not calling for it to be applied because it's not an Islamic state.'

Hizb ut-Tahrir spokesman Uthman Badar

Hizb ut-Tahrir wants a global Islamic caliphate based on Sharia law and has a draft constitution for the khilafah state which says ex-Muslims should be killed.

'Those who are guilty of apostasy from Islam are to be executed according to the rule of apostasy, provided they have by themselves renounced Islam,' Section 7c of this draft constitution says.

At Bankstown library in Sydney's west in March, Daily Mail Australia caught Mr Badar on camera confirming his support for giving ex-Muslims capital punishment.

'The ruling for apostates as such in Islam is clear, that apostates attract capital punishment and we don't shy away from that,' he said.

Pakistani-born extremist Uthman Badar wants capital punishment for ex-Muslims but only where Sharia law applied

Daily Mail Australia did not infer in March that Mr Badar wanted ex-Muslims killed in Australia.

Despite that, the Pakistani-born extremist accused Dr Perkins of relying on an inaccurate Daily Mail Australia story.

'On the question of apostates, the point here is John's information is from the Daily Mail,' Mr Badar said.

'I don't know why anyone reads the Daily Mail in the first place. It used to be The Daily Telegraph, and this was the worst of the worst.'

Secular Party of Australia president John Perkins asked Uthman Badar to answer him

After that Thursday night debate, organised by the Sydney University Muslim Students' Association, Dr Perkins called out Mr Badar for evading the question.

'He didn't really answer the question,' he told Daily Mail Australia.

Dr Perkins, an economist, defended his decision to participate in the debate at the University of Sydney's Footbridge Theatre.

'I believe in freedom of speech, we should give them enough rope,' he said.

A range of Muslim-majority nations impose capital punishment for leaving Islam.

Atheist activist John Perkins said the Koran incited violence and asked about killing ex-Muslims

Secular Party of Australia president John Perkins said Uthman Badar evaded his question

They include Iran, Nigeria, Somalia, Sudan, Pakistan, Yemen and the United Arab Emirates.

Dr Perkins said the Koran was an incitement to violence.

'You can argue that the Koran is an incitement to violence because it calls for violence and all Muslims are supposed to follow the words in the Koran,' he said.

Secular-minded Muslims, who accept the separation of religion and state, reject Sharia law and any fundamentalist interpretation of the Koran.

Hizb ut-Tahrir is so extreme it is banned in Indonesia, the world's biggest Muslim-majority nation, along with other Islamic nations including Bangladesh, Pakistan, Saudi Arabia and Uzbekistan.

It is also illegal in The Netherlands and Germany.

However, Australia's Attorney-General George Brandis has declined to ban Hizb ut-Tahrir on the grounds it hasn't committed any violent acts in Australia.