Muslim leaders have accused far-right extremists of trying to capitalise on the “sick and barbaric” murder of Lee Rigby to fuel racial hatred.

Islamophobic hate crimes are running at more than 10 times their usual rate, with more than 140 reported to a government-backed hotline in the 48 hours since the Woolwich killing.

They include nine attacks on mosques, assaults, racial abuse and anti-Muslim graffiti. An improvised petrol bomb was thrown at a mosque in Milton Keynes during Friday prayers, while attacks have also been reported in Gillingham, Braintree, Bolton and Cambridge.

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The British National Party leader, Nick Griffin, who visited Woolwich yesterday, provoked widespread disgust for tweeting that the alleged killers should be wrapped in “pig skin” and shot again. The English Defence League, which has said the killing shows Britain is “at war” with Islamic extremism, staged a march today in Newcastle, sparked by plans to open an Islamic school.

The march was already scheduled before Wednesday's murder.

Police arrested two people from Gateshead and another from Stockton, Teesside, ahead of today's march, for allegedly making racist tweets.

More than 1500 people marched with the EDL - three times as many as was expected earlier this month. Chants of "Whose streets? Our streets" rang out, as well as "RIP Lee Rigby." Flags from Teesside, Coventry and Bournemouth were on display, along with one marked "Taliban Hunting Club."

Meanwhile, a nearly 400-strong counter-demonstration called Newcastle Unites, which was separated from the EDL by a police line sang: "Nazi scum, off our streets."

Chief Superintendent Gary Calvert said there had been "a number of arrests" during the day, mainly for drunkenness or to prevent public order offences, but there was no major trouble. The EDL is also planning a demonstration in central London on Monday.

Amid heavy security, Mr Griffin visited the site of the killing following a series of provocative tweets in which he claimed the attack was the result of “mass immigration”.

The BNP leader, whose party’s electoral support has collapsed in the past three years, has also called for a show of strength by activists in Woolwich next Saturday under the banner “United against Muslim terror”.

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In an unprecedented open letter, the heads of nearly 100 mosques said they shared the “absolute horror” felt by the rest of British society at the crime committed “in the name of our religion”.

But they warned that across Britain “hate-fuelled individuals” had already attempted to attack mosques and individuals in the wake of Wednesday’s killing. They urged the public “not to be taken in” by the “mindless rantings” of extremists on both sides who, they said, should be “isolated and subject to the full force of the law”.

The Muslim leaders said they wanted to make clear that the murder of Drummer Rigby was a “heinous atrocity worthy of nothing but contempt”. But they warned they had already seen extremists “seeking to capitalise upon Wednesday’s terrible act”.

“The hate-fuelled individuals behind such attacks wish to polarise and tear apart our great country for their own sick ends,” they wrote. “They should be isolated and subject to the full force of the law.”

A spokesman for the hate crime hotline Measuring Anti-Muslim Attacks said reported incidents were running at “a level we simply haven’t seen before”. He said: “Muslims at this moment are feeling a real and pervasive sense of fear.” Calls for action on the capital’s streets by the EDL and the BNP will leave the Metropolitan Police Commissioner, Sir Bernard Hogan-Howe, with the dilemma of whether to apply to the Home Secretary, Theresa May, to have them banned. A planned EDL march through Tower Hamlets in London, home to one of the country’s largest Muslim communities, was blocked by Ms May two years ago.

Far-right websites are linking the murder to population growth among ethnic minorities, while a number of social networking sites also carried messages calling for Muslim sites to be attacked. The “True British Patriots” Facebook page carried calls for mosques to be burned down. The official website of the National Front party berates “Muslim scum”.

Sunder Katwala, director of the British Future think-tank, said: “The BNP and EDL, both in a state of near collapse, have little chance of using Woolwich to recover politically but their activities do often stir up local violence. Yet again the absurdly extreme Islamist clown Anjem Choudary shows he is a more effective recruiter for the far right than Nick Griffin has ever been. How much these two extremes need each other.”

In a speech in London, alongside representatives of the Army and the Muslim community, the Deputy Prime Minister Nick Clegg expressed fears that the Woolwich atrocity could lead to long-lasting damage to community relations. “Fear is an extraordinarily powerful emotion and when it takes root,” he said, “it has a very, very corrosive effect on every part of our lives. We have a choice to either allow that powerful corrosive feeling of fear to seep into every second and minute and hour of our lives or we can make a choice that we’re not going to change our behaviour.”

The Archbishop of Canterbury called for community unity. Speaking after a meeting of faith leaders in Leicester, the Most Rev Justin Welby said: “I want to recognise the response of churches, mosques and other faith and civil society groups as well as those of brave individuals who have done so much to bring our communities together at this time.

* A 22-year-old man will appear in court today after being arrested on suspicion of making malicious comments on Facebook following the murder of Lee Rigby. Benjamin Flatters, of Lincoln, was arrested after complaints made to Lincolnshire Police that alleged the comments were of a racist or anti-religious nature.