Millions of people have been marching through Paris in solidarity with the 17 people who died at the hands of Islamists terrorists. Thousands of police officers are on duty and France remains on its highest terror alert following the attacks.
In London, famous landmarks, including Tower Bridge have been lit up in the tricolore colours. The water fountains on Trafalgar Square are illuminated in red, white and blue and a French flag has been projected onto the National Gallery.
Earlier, British Prime Minister David Cameron travelled to Paris to join the march "to show solidarity with the French people".
At the same time in London, hundreds of people gathered on Trafalgar Square clutching pens, holding signs or waving a flag. Smiles turned into visible expressions of determination and unity on the faces of those who had congregated. People spilled from the steps of the National Gallery down to Nelson's Column; sporadic bursts of applause and singing infrequently interjected the solitude.
Many in cities across the UK took to the streets carrying 'Je Suis Charlie' placards. Glasgow in Scotland was no exception, as hundreds demonstrated in solidarity and paid tribute to the lives lost in Paris this week.
Former security minister Lord West says intelligence agencies "need more money" but adds threat to UK "not immensely greater" #marrshow— PoliticsHome (@politicshome) January 11, 2015
German tabloid Bild quotes intelligence sources and experts who believe Europe is facing a fresh wave of terrorism.
Home Secretary Theresa May held meeting with her EU counterparts on what lessons can be learned following the terror attacks in Paris; David Cameron is meeting with security and intelligence chiefs on Monday.
Meanwhile, Theresa May has ordered border guards and police to ramp up their efforts to find illegal firearms.