A British man who claims he has been fighting in Syria says he will not return until the "black flag of Islam" is flying over Buckingham Palace.
He said he had been fighting alongside the al-Nusra Front, which is linked to al-Qaeda and banned in the UK.
Security services estimate 500 Britons have gone to Syria to fight.
It comes as Islamic leaders urge British Muslims not to go to Syria or Iraq amid fears people are travelling there to take part in fighting.
Speaking to BBC Radio 5 live, the man - who spoke in a West Yorkshire accent and called himself Abu Osama - described Britain as "pure evil".
Sir Peter Fahy, who heads Association of Chief Police Officers' (Acpo) "prevent" strategy on counter-terrorism, told Radio 5 live that young Muslims - including girls - were being brainwashed at an "impressionable" age.
"We have got information about some young girls trying to get to Syria - indeed some we believe might have got out there - on the idea of being a jihad bride," he said.
The man calling himself Abu Osama said he had been fighting in Syria for 12 months alongside groups such as the al-Nusra Front, although his claims cannot be officially verified.
"If and when I come back to Britain it will be when this Khilafah, the Islamic state, comes to conquer Britain, and I come to raise the black flag of Islam over Downing Street, over Buckingham Palace, over Tower Bridge and over Big Ben," he added.
The man said he initially faced opposition from his family over his decision to go to Syria.
"At first it was hard for them to accept, because no mother wants to lose her son," he said.
"But I send them photos and I speak to them about the situation and now they can understand that this is a good cause I am in.
"They are a bit scared but I tell them we will meet in the afterlife. This is just a temporary separation.
"They said, 'We understand now what you are doing,' and my mother said, 'I have sold you to Allah. I don't want to see you again in this world'."
He told the BBC he had been taking part in military training, making bombs and shooting enemies.
In April, the Metropolitan Police issued a plea for people to come forward with information about their family members if they were concerned about them joining terrorist training camps in Syria.
More than 100 imams have signed an open letter urging Muslims to offer help "from the UK in a safe and responsible way".
In the open letter released to coincide with the holy month of Ramadan, they said: "As the crisis in Syria and Iraq deepens, we the under-signed have come together as a unified voice to urge the British Muslim communities not to fall prey to any form of sectarian divisions or social discord.
"Ramadan, the month of mercy, teaches us the value of unity and perseverance and we urge the British Muslim communities to continue the generous and tireless efforts to support all of those affected by the crisis in Syria and unfolding events in Iraq, but to do so from the UK in a safe and responsible way."
Speaking to BBC Radio 4's Today programme, Qari Mohammed Asim, an imam in Leeds, said the group had come together in a bid to send a "very strong message" to British Muslims thinking of travelling to Syria or Iraq.
"I think a lot of work needs to be done and it is not only the responsibility of the Muslim community or the imams.
"It is law enforcement, (and) intelligence services who all need to work together to make sure young British Muslims are not preyed upon by those who want to use them for their own political gains."
Meanwhile, BBC Newsnight has learned that the UK drew up plans to train and equip a 100,000-strong Syrian rebel army to defeat President Bashar al-Assad, but the initiative devised two years ago was considered too risky.
Concerns have been raised about home-grown involvement in terrorism, after Britons appeared in a propaganda video for insurgent group Islamic State in Iraq and the Levant (Isis).
In the video Reyaad Khan, who went to Cantonian High School and St David's Catholic College in Cardiff, is seen holding a gun and sitting next to his friend Nasser Muthana.
Nasser Muthana - who has been offered places to study medicine by four universities - appears in the footage, using the name Abu Muthanna al-Yemen and urging others to fight in Syria and Iraq. A third Briton in the video is from Aberdeen, where he has been named locally as Raqib.