Britain was placed on high alert on Saturday as Prime Minister David Cameron chaired an emergency security meeting following Friday's killings in Tunisia in which most of the holidaying 38 victims were said to be British.
There was heightened security during two major public events on Saturday: a flag march during the Armed Forces Day and the gay community's Pride in London event attended by tens of thousands of people.
The mass circulation tabloid, The Sun, reported on Saturday that it had foiled a terror attack on soldiers participating in the Armed Forces Day parade, when a person linked to Islamic State reportedly recruited an undercover investigator, unwittingly, to detonate a bomb during the parade.
The plot was reportedly to target the same army unit of fusilier Lee Rigby, who was killed in May 2013. Scotland Yard urged the public to attend public events as normal on Saturday.
Cameron said, “These terrorists will not succeed. For as much as they try to divide people around the world, they will only unite us more strongly in our determination to defeat these Islamist extremists and all that they stand for."
He said the Tunisia attack was "savage" and called on the British people to work together with the authorities to "combat this poisonous narrative of Islamist extremism that is radicalising too many young minds in our country".
Travel agents Thomson and First Choice said their customers were among "a number of fatalities", and they had sent 10 planes to bring home 2,500 tourists.
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. While the UK threat level from international terrorism remains severe, we would like to reassure the public that we constantly review security plans for public events, taking into account specific intelligence and the wider threat”, he added.