Prime Minister David Cameron paid tribute to service personnel as events were held across the country to mark Armed Forces Day.
The PM attended the main celebration in Guildford, where a service was held at the cathedral.
Security at the events was heightened after attacks in Tunisia, France and Kuwait on Friday.
The Sun also reported that so-called Islamic State had planned to target a parade in Merton, South London.
The newspaper said IS had plotted to detonate a pressure cooker bomb, but the plot failed after it informed police and security services.
Police did not confirm the Sun's report but said it was "helpful" when journalists shared information that could indicate terrorist activity.
Speaking in Guildford, Mr Cameron said: "Today, we celebrate our brave servicemen and women who work so hard to keep our country safe."
Defence Secretary Michael Fallon and Vice Chief of the Defence Staff Air Chief Marshal Sir Stuart Peach also attended the event.
The Duke of York took the salute on the parade route on behalf of the Queen and Royal Family, as the Red Arrows staged a fly-past.
More than 150 events took place across the country to mark the seventh annual celebration.
Mr Fallon told the BBC Armed Forces Day was a "huge opportunity to salute the armed forces for the service and the sacrifice they make on behalf of us all and also to pay tribute to families who support them".
Earlier the PM chaired an emergency Cobra meeting to look at the UK's response to the attack in Tunisia.
At least five Britons have been confirmed dead by the Foreign Office following the attack on Friday, but Mr Cameron warned the public must be prepared for a high British death toll. The Tunisian health ministry said eight Britons had been identified among the dead.
Following the Cobra meeting Mr Cameron said: "Let's be clear. There's no doubt we face a very severe threat in our country and we have done for many months and many years...
"But today is about paying tribute to our armed forces and their families who do so much to keep us safe and we should proceed with those events."
'Suicide attack'
Meanwhile, the Sun reported that an IS leader in Syria unwittingly recruited an undercover investigator from the newspaper to carry out the attack in Merton.
The Sun described the alleged plot as a "suicide" attack, saying IS was intending to strike soldiers from the unit of murdered soldier Lee Rigby.
It alleged an IS leader, who it said was originally from Birmingham, told the investigator: "It will be big. We will hit the kuffar (unbelievers) hard InshAllah. Hit their soldiers in their own land."
The Merton parade was to be targeted because it was closest to the barracks in Woolwich where Fusilier Rigby, was murdered in May 2013, the newspaper said. Fusiliers from his regiment were to be among some 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."
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