A benefit concert for the Manchester bombing victims will still take place but security will be stepped up following the London attack, police have said.
Ariana Grande said she was "praying for London" ahead of the concert on Sunday night, which will see her return to the stage for the first time since suicide bomber Salman Abedi detonated a device, killing 22 of her fans and parents as they left her show at the Manchester Arena on May 22.
The One Love Manchester event at Lancashire Cricket Club's Old Trafford ground will take place less than 24 hours after seven people died in the London Bridge attack - the country's second terrorist outrage in less than two weeks.
Assistant Chief Constable Garry Shewan from Greater Manchester Police said: "We're deeply saddened to hear about last night's horrific attacks in London and our thoughts are with everyone affected, including the emergency services responding to the incident.
"There are two large-scale events taking place in Greater Manchester today and we would like to assure people that these will still take place, but with additional security in place to ensure the safety of everyone.
"We have dedicated resources at both events, with a significant number of officers from both GMP and colleagues from other forces, some of which will be armed.
"There will be additional security checks taking place and everyone will be searched, including bags. We would ask people not to bring bags if they can, as this will help speed up entry.
"I'd like to remind people that the threat level remains at severe, which means an attack is highly likely."
He warned people to remain vigilant at all times.
A charity testimonial for Manchester United player Michael Carrick is due to take place at Old Trafford football ground this afternoon.
The kick off time was moved from 4pm to 2.30pm to accommodate the concert.
Following the London attack Grande tweeted an emoji of a heart, and wrote: "Praying for London."
Other musicians including Justin Bieber, Katy Perry, Coldplay, Take That and Miley Cyrus are to join Grande for the Sunday night event.
Grande has visited victims of the attack at the Royal Manchester Children’s Hospital ahead of the show, where she will also be joined by Robbie Williams, Little Mix, Usher, Pharrell Williams and One Direction star Niall Horan.
Marcus Mumford, frontman of Mumford & Sons, is also joining the all-star line-up for the benefit concert, it was reported.
Proceeds from the concert will go towards the We Love Manchester Emergency Fund, set up by Manchester City Council in partnership with the British Red Cross, to support grieving families and victims of the bombing.
All Grande fans who attended the gig on May 22 were offered free passes to Sunday's benefit concert regardless of where they bought their tickets.
The event will be broadcast live on BBC One, BBC Radio 1, BBC Radio 2, BBC Radio 5 live, and BBC Radio Manchester, as well as commercial radio.
Millions of people around the world will also be able to watch as the concert will be simulcast in more than 50 countries across five continents.
:: The One Love Manchester Concert will be shown on BBC One from 6.55pm.
Grande's manager Scooter Braun has said the One Love Manchester concert now has a "greater purpose" following the London terror attack, and that all the acts involved are showing "unwavering" support.
He said in a statement: "After the events last night in London, and those in Manchester just two weeks ago, we feel a sense of responsibility to honour those lost, injured, and affected.
"We plan to honour them with courage, bravery, and defiance in the face of fear. Today's One Love Manchester benefit concert will not only continue, but will do so with greater purpose.
"We must not be afraid and in tribute to all those affected here and around the world, we will bring our voices together and sing loudly.
"I am pleased to say we have the full support of Greater Manchester Police and the Government and are assured the safety of all those attending is the highest priority.
"All artists involved have been unwavering in their support this morning and are determined to carry on with the show. We ask the strong city of Manchester and the world to join us in making the statement that hatred and fear will never win.
"Today we stand together. Thank you."