BIRMINGHAM, UNITED KINGDOM - JANUARY 31: Police officers guard a house in Poplar Rd after a series of anti-terror dawn raids across the city resulted in the arrest of eight men, on January 31, 2007 in Birmingham, England. It is alleged that a kidnapping plot to mirror that of terrorist kidnappings in Iraq was planned. (Photo by Christopher Furlong/Getty Images)

Four men have been arrested in raids across London on suspicion of preparing to commit an act of terrorism in the UK in one of the most serious Syria-linked security operations this year.

Several properties across the capital are being searched by officers from the Metropolitan Police’s counter terrorism branch. Scotland Yard’s armed response squad, SCO19, was called to one of the addresses. No shots were fired but one of the men arrested was tasered while trying to escape.

At least one of the men arrested has previously travelled to Syria to fight with Islamist extremists, a security official told the Financial Times.

The group of men are suspected of being linked to an “early stage” plan to conduct an attack in the UK, the official said.

The UK Security Service, MI5, raised the terrorism threat rating to severe – the second highest level – in August over fears of the threat posed by Syrian jihadis.

About 500 British citizens have travelled to Syria to fight with extremist groups. Most have joined the Islamic State of Iraq and the Levant, known as Isis.

According to prime minister David Cameron, Isis is plotting terror attacks against British targets.

Last year police arrested a number of Britons linked to Syrian groups who had been plotting a “Mumbai-style” terror atrocity in London.

So far, only one Syria-linked terror plot has succeeded. Mehdi Nemmouche, a Frenchman, murdered four visitors to a Belgian Jewish museum in May this year. Mr Nemmouche had previously fought with Isis in Syria.

The Metropolitan Police said: “A number of residential addresses and vehicles are being searched by specialist officers in west and central London as part of the investigation. The searches are ongoing.”

“These arrests and searches are part of an ongoing investigation into Islamist-related terrorism,” it added.

Two weeks ago 11 men were arrested in an operation, including radical preacher Anjem Choudary, but they were later released on bail. Their arrests were not connected with plotting to commit acts of terror, security officials said.

British security authorities have ramped up their efforts to curb extremism in the UK significantly in recent months.

Theresa May, the home secretary, promised sweeping new anti-terror powers in the next parliament if the Conservative party is returned to power, including measures to gag known extremists and prevent them from publishing or publicising their views even if they are not specific exhortations to violence.

More terrorism-linked extremist content has been removed by UK security authorities from the web in the past year than in the past four years combined.

British participation in a US-led campaign of air strikes begun in August against Isis has recently led to jihadis specifically threatening Britain with reprisals. Two Britons – aid workers David Haines and Alan Henning – have been murdered in the past month by Isis in propaganda videos that criticised British military intervention. The killer – who appears masked in the videos – is also suspected of being a Briton. US and UK authorities are understood to know his identity but they have not made it public. The latest propaganda video – released on Friday – threatens a US hostage, Peter Kassig, with execution if air strikes continue.

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