London, March 29 (IANS) A group of extremist Muslims has allegedly threatened to disrupt the wedding of Prince William, saying "the day the nation has been dreaming of will become a nightmare".

William, 28, will marry Kate Middleton, 29, at Westminster Abbey April 29.

A Muslim group - named Muslims Against Crusades - vowed to disrupt the royal wedding and even threatened to start a riot during the celebrations, The Sun reported citing Scotland Yard officials.

Meanwhile, a right-wing group, the English Defence League, has also vowed to act against people who might try to wreck the royal event.

The Muslim group has set up a website showing pictures of Prince Harry with a tagline saying "Bestman is a Nazi". It was referring to a 2005 photograph showing Harry at a friend's birthday wearing a Nazi armband.

The site termed the royal family "enemies to Allah and his messenger".

The British flag is shown in flames along with pictures of Muslim children who the group claims were killed by British soldiers.

The site asks William and Harry to "withdraw from the British military and give up all affiliation to the tyrannical British Empire".

"We promise that should they refuse, the day the nation has been dreaming of will become a nightmare," it says.

Police have said they could use stop-and-search powers to stop wedding protesters and guard against any terrorist threat.