Armed Forces Day passed off peacefully, with hundreds of service personnel marching in events across the country, amid reports of a foiled plot to targeted a parade in south west London.

Events to mark the day took place as planned, including a celebration in Guildford attended by Prime Minister David Cameron.

Police had encouraged the public to attend events as normal after a supposed plot by Islamist fighters to attack a parade in Merton was foiled when the would-be attackers recruited an undercover reporter from The Sun newspaper to carry it out.

The Sun reported that the plan was to explode a pressure cooker bomb in order to kill soldiers and bystanders on the route of the parade. But the man trained to carry out the attack was an undercover reporter for the paper.

The report of the foiled attack comes amid a series of terror attacks by Islamist fighters internationally.

The Sun said that Junaid Hussain, who it described as an Isis "kingpin", had "trained our man to build the bomb, but we alerted police".

Hussain, a computer hacker linked to Isis, left the UK for Syria in 2013. The term "kingpin" may present an overly generous assessment of Hussain's rank within the violent Islamist group.

The parade was targeted because it was closest to the barracks in Woolwich, south east London, where Fusilier Lee Rigby, 25, was murdered by Islamist extremists in 2013, the newspaper said.

Fusilier Lee Rigby was murdered by Islamist extremists in 2013 (Getty)

Fusiliers from his regiment, serving Gurkhas and war veterans will be among the 250 marchers, it added.

A Scotland Yard spokesman said: "The police, together with our security partners, remain alert to terrorist threats that may manifest here or where individuals overseas may seek to direct or inspire others to commit attacks in and against the UK.

"It is always helpful when journalists share with us information, as The Sun did in this case, that could indicate terrorist or criminal activity."

There may have been an indication of possible heightened tensions among the armed forces over the bomb plot when a member of the Queen's Guard pointed his rifle at a tourist after he clapped his hand on his shoulder.