The event, which begins at 6pm, is open to all Londoners and anyone visiting the city.
Flags will also remain at half-mast on Whitehall government buildings, Downing Street has announced, while the country mourns the victims of Saturday night’s devastating terror attack.
In a message on the Mayor of London’s website to everyone in the capital, Sadiq Khan said: “The Mayor invites all Londoners – and everyone visiting our city – to come together in solidarity to remember those who have lost their lives in Saturday’s attack, to express sympathy with their families and loved ones and to show the world that we stand united in the face of those who seek to harm us and our way of life.
“We will never let these cowards win and we will never be cowed by terrorism.
“Members of the public will be able to lay floral tributes at the flagpoles at City Hall.”
Flags across the country have already flown at half mast after the terror attack, where the attackers ploughed a van into pedestrians walking on London Bridge before moving through restaurants and bars in Borough Market to indiscriminately stab and kill innocent civilians.
Britain will also observe a minute’s silence on Tuesday in memory of the victims of the terror attack on London Bridge, which left seven people dead and 21 more fighting for their lives Saturday evening.
The nationwide silence will be held at 11am.
The attack is the third terror incident in Britain since March, after Khalid Masood killed five people in a similar attack on Westminster Bridge and less than fortnight ago, Salman Abedi murdered 22 people with a suicide bomb at an Ariana Grande concert in Manchester.
Islamic State claimed responsibility for the London terror attack.
Though he has not yet been named, one of the attackers is said to have been radicalised by watching YouTube videos of the American hate preacher Ahmad Musa Jibril.
A former friend said he contacted the place after the man began to express increasingly radical views, and justifications for terror attacks.
Neighbours of the man in Barking, east London – where police carried out a raid Sunday – said he was of Middle Eastern or Pakistani origin.