Manchester attack: Soldiers may be deployed to FA Cup final
Soldiers could be deployed to events including the Chelsea Flower Show and the FA Cup final after the UK terror threat level was raised to critical.
Armed troops are already being deployed to protect key locations such as Buckingham Palace, Downing Street and embassies to free up police officers.
It comes after the bomb attack at Manchester Arena on Monday which killed 22 people, some as young as eight.
London mayor Sadiq Khan said security experts were advising event organisers.
He said there was "no reason to be alarmed or to panic, this is a prudent course to take".
The mayor added he was in close contact with Met Police Commissioner Cressida Dick and the force had assured him they have all the resources they need.
The decision to replace police officers with soldiers at key public locations was taken at a meeting of the government's emergency Cobra committee on Tuesday.
The Palace of Westminster was shut to the public and all tours and events due to take place there have been cancelled.
Ariana Grande's concerts at London's O2 Arena on Thursday and Friday have also been cancelled following the bombing.
A statement said the US singer's Dangerous Woman tour would be suspended until 5 June.
A man with a knife was arrested outside Buckingham Palace earlier, shortly before the Queen was driven along the Mall. The Met said the incident was not terror-related.
Elsewhere, a man has been arrested in Vicarage Fields, Barking, for obstructing a terrorism search and assaulting police.
In the wake of the attack, Chelsea have also cancelled their Premier League victory parade, which was due to take place on Sunday.
"In light of these tragic events, we feel it is inappropriate to go ahead with the victory parade in London on Sunday", the club said.
"Given the heightened security threat announced by the Government, and recognising that this is a developing situation, we have given this careful consideration."
The London premiere of the new Wonder Woman film due to take place on 31 May has also been cancelled. The film is released worldwide on 2 June.
The FA Cup final between Arsenal and Chelsea takes place at Wembley on Saturday - the final day of the Chelsea Flower Show.
Commander Jane Connors, leading the London policing operation, said: "We will do all we can to protect the capital at this unsettling time. All our work is designed to make our city as hostile an environment as possible for terrorists to plan and operate."
Other measures being taken include an increase in armed officers to provide support for uniformed and undercover officers trying to spot people carrying out "hostile reconnaissance" and other criminal activity in London.
The tactic is "based on extensive research into the psychology of criminals and what undermines their activities", the force said.
'Will use force'
There will also be an increase in the use of stop and search, vehicle check points and number plate recognition technology.
All waste bins and mobile phone charging points have been removed from the the Centrale in Croydon.
The main car park at Brent Cross shopping centre has also been closed for security reasons while bag searches at the entrance of the centre are causing hour long queues.
"The reality is we must be prepared to be able to respond to and deal with armed and deadly attackers, so we must be in a position to respond with firearms officers who will use force to stop those attackers in their aim," added Ms Connors.