London landmarks will be lit in the colours of the French national flag later in tribute to those murdered in last week's attacks in Paris.
Locations lit in blue, white and red will include Tower Bridge, Trafalgar Square, and the National Gallery.
In Paris Prime Minister David Cameron will be among politicians from around the world joining a unity rally.
And Home Secretary Theresa May will join other EU interior ministers to discuss the response to the attacks.
London mayor Boris Johnson said it was important that London demonstrated its solidarity with Parisians.
"Londoners have been appalled by the distressing scenes in France this week and it is important that we pay tribute to the victims of these attacks, as well as demonstrate our solidarity with the people of Paris.
"We should not forget the hundreds of thousands of French citizens who call London their home. Our thoughts will be with them this weekend as they do their best to deal with the agonising scenes we have witnessed."
Tower Bridge will display the flag's colours from 16:00 GMT until 17:30 GMT, after which the bridge will go dark.
The emblem will also be projected on to the National Gallery.
The Trafalgar Square fountains will rotate the colours of the French flag, while the London Eye will go dark with the French colours projected onto County Hall behind it.
The Paris offices of magazine Charlie Hebdo were attacked by brothers Cherif and Said Kouachi on Wednesday in an act of Islamic extremism.
Twelve people - including eight journalists and two police officers - were killed and 11 injured in that attack.
The brothers then went on the run, before becoming surrounded by police at a printworks warehouse in Dammartin-en-Goele, 35km (22 miles) north of Paris, on Friday. They were eventually killed after exchanging fire with police.
Another gunman, Amedy Coulibaly, took several hostages at a kosher supermarket in eastern Paris on Friday. Four hostages were later found dead.
Police stormed the supermarket on Friday evening, killing Coulibaly and rescuing 15 hostages.
Coulibaly is also believed to be behind the killing of a policewoman in southern Paris on Thursday.
International presence
Sunday's mass rally in Paris is expected to be attended by more than one million people, with national leaders and their representatives from around the world flying in to take part.
These include German Chancellor Angela Merkel, Canadian Public Safety Minister Steven Blaney, Italian Prime Minister Matteo Renzi, Russian Foreign Minister Sergei Lavrov, European Union President Donald Tusk and NATO Secretary General Jens Stoltenberg.
And about 700,000 people are said to have taken part in marches across France on Saturday.
They were held in cities including Paris, Orleans, Nice, Pau, Toulouse and Nantes, to commemorate the victims of this week's violence.
(BBC World)
