Extra armed police officers will be deployed at Wembley for this weekend's FA Cup Final and at Twickenham for the Aviva Premiership final, Scotland Yard said.
A full review of the security and policing operations for the weekend's sporting events - the climax of the football and rugby season - is under way in the aftermath of the Manchester suicide bombing and as Britain stands on a severe terror threat level.
Metropolitan Police Commander Jane Connors said: "Here in London we are determined to do all we can to protect the capital.
"That means that over the coming days as you go to a music venue, go shopping, travel to work or head off to the fantastic sporting events, you will see more officers - including armed officers.
"We are used to delivering policing operations that seek to mitigate against the severe threat we face from terrorists.
"It is only right that we now take time to review those existing plans and make sure we are confident that we are doing all we can."
Wembley Stadium officials said there will be "an enhanced security operation for all upcoming events".
Wembley will host the FA Cup final between Arsenal and Chelsea on Saturday, the League Two play-off final between Blackpool and Exeter on Sunday and the Championship play-off final between Huddersfield and Reading on Monday.
A Football Association spokesman said: "Fan safety is of paramount importance and we have robust security measures in place at Wembley Stadium.
"In collaboration with the Metropolitan Police and the local authorities there will be an enhanced security operation for all upcoming events.
"All supporters are encouraged to arrive for events at Wembley Stadium as early as possible for security checks and to avoid any delays in entering the stadium."
Officials for the Wimbledon tennis tournament, which is due to start in south-west London on July 3, have also said that plans are in place to ensure the safety of the Grand Slam event.
A spokesman for the All England Lawn Tennis Club (AELTC), which hosts the Wimbledon championships, said: "The safety and security of all our visitors is of paramount importance.
"Throughout the year, the AELTC works closely with the Metropolitan Police Service and other relevant organisations to ensure that The Championships are as secure as possible and that our plans are commensurate with the existing threat level in London and the UK.
"We will take any appropriate measures to ensure the safety of the whole Championships."
Scotland Yard said that Operation Hercules, involving the use of mobile and static overt armed officers, will be stepped up to protect London against would-be attackers.
Specialist officers are now reviewing in detail plans for all upcoming events in London, including smaller events which previously may not have been of police interest.
Licensing officers are also being urged to keep their music events as safe as possible.
The Chelsea Flower Show, which expects to see 165,000 visitors pass through its gates by the time the event closes on Saturday, said it is maintaining a "strong security presence".
A Royal Horticultural Society (RHS) spokesman said: "We have a very strong security presence at RHS Chelsea Flower Show and work closely with the police and emergency services throughout the year to ensure an appropriate policing plan is in place."
The assurances followed the suicide bombing which killed 22 people, including children, at Manchester Arena shortly after US pop star Ariana Grande finished her performance on Monday night.
A spokesman for the O2 arena in Greenwich, south-east London, said: "Along with everyone else, we are shocked and saddened by the terrible tragedy in Manchester.
"Our thoughts and prayers are with those affected and their families.
"Ariana is due to perform at The O2 on Thursday and Friday and we will advise as soon as we can as to the status of those shows."
The O2 also said anyone visiting the venue should allow extra time for enhanced security checks.
This would include tickets being checked as well as bag searches and going through a screening arch. Large bags are not permitted.
A Rugby Football Union (RFU) spokesman said: "We have been operating enhanced security measures for some time and will continue to do so for both match days this weekend. We work closely with the police for all our events at the stadium.
"Any fans planning their trip should allow extra time to access the stadium and avoid bringing bags where possible."