An extremist Muslim group whose request to protest outside Westminster Abbey during the royal wedding has been turned down has pledged to turn the day into a "nightmare".
Muslims Against Crusades, whose website features images of the Union Jack and the crown burning, had requested permission to hold a static demonstration outside the wedding venue on April 29th.
Metropolitan police assistant commissioner Lynne Owens told journalists at a briefing about security measures that although negotiations were ongoing the request had been rejected.
A statement on the group's website threatened unspecified actions if its "forceful demonstration" was not permitted.
"We promise that should they refuse, then the day which the nation has been dreaming of for so long will become a nightmare and that it will inshaa'allah (God willing) eclipse the protests in Barking, Downing Street and the events of November 11th," it concluded.
The group has already attracted condemnation after a member burned poppies at an Armistice Day ceremony. Emdadur Choudary, 26, was fined £50 as a result.
Muslims Against Crusades criticises Britain's "quest to occupy Muslim land and wage war against the religion of God (Allah)", as well as "the forced indoctrination of the satanic democratic creed".
Ms Ownes said the royal family's "celebration" would not be disrupted by any protests, however.
"What we have to do whenever we're deciding about whether there can be a protest, if they're asking to protest in the area around parliament, it is defined by a very set definition of law - we have to authorise a demonstration but we can put conditions on that demonstration," she told the BBC.
"It's that negotiation process that we're engaged in at the moment. But [people] should be absolutely reassured that it won't disrupt the day and we'll have a very safe and happy celebratory event."
The far-right English Defence League has requested to hold a counter-protest at the same time.
Ghaffar Hussain, head of training and outreach at counter-extremism thinktank Quilliam, said news of the potential clash was "deeply disappointing" but also "entirely predictable".
"The royal wedding celebrations now risk being hijacked by a few publicity-hungry fanatics whose only aim is to divide our society and to sow suspicions and hatred between Britain's different peoples," he commented.
"These protests show the urgent need for government strategy to move ahead on tackling systematically all forms of extremism across society - and to recognise that Islamist extremism is currently fuelling right-wing extremism and vice-versa. For too long such extremism has been left to fester in Britain unchecked."
The Met will deploy 5,000 police officers during the royal wedding as well as up to 80 additional officers on close protection duty for key figures.