Famous landmarks around the world were bathed in red, white and blue in a show of international support for France.
It comes as Britain braces for copycat attacks on UK soil.
But on the streets of London they were united and defiant. Thousands gathered in Trafalgar Square to attend a vigil in honour of those who were killed.
A modern candlelight vigil including cellphone torches – the new flame – were used to pay respect to the dead.
Thousands of mourners gathered in the rain holding up placards saying "Je suis Paris".
Even British Prime Minister David Cameron spoke in French to express his support.
"My message to the French people is clear – we stand with you united. Your values are our values, your pain is our pain, your fight is our fight and together we will defeat these terrorists," he said.
The French flag was projected onto the National Gallery's façade in front of them, where they stood in solidarity.
Landmarks throughout the capital, throughout the world, turned red, white and blue, illuminated in the colours of the French flag.
There were other visual clues too, reminders of the attack just across the channel.
There was a strengthened police presence in the UK, and Mr Cameron warned of possible attacks on British soil.
"We must recognise that however strong we are, however much we prepare, we in the UK face the same threat. That's why we continue to encourage the public to remain vigilant and we will do all we can to support our police and intelligence agencies with the resources and capabilities that they need," says Mr Cameron.
The first scare came just hours later, when police arrested a man and evacuated part of Gatwick Airport after a firearm was discarded in a bin at the north terminal.
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