Unity March: London Landmarks Wear Tricolore

As British PM David Cameron took part in a Unity March alongside Francois Hollande and other world leaders in Paris today, crowds gathered in Trafalgar Square to pay tribute to those killed this week, as well as witness London's landmarks light up in the colours of the French national flag.

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.

.@Reuters photo: London's Tower Bridge lit in colors of French flag to honor those killed in Paris attacks pic.twitter.com/EKkl8Cy8TE

— BuzzFeed News (@BuzzFeedNews) January 11, 2015

Solidarity at Trafalgar Square #JeSuisCharlie #JeSuisAhmed pic.twitter.com/wU5KVKQQQH

— Gavin Hansford (@djgav) January 11, 2015

Amazing scenes in Trafalgar Square #JeSuisCharlie pic.twitter.com/XkkhmFn0Aj

— Neil Hurst (@neilhurst) January 11, 2015

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.

Crowds gather in London's Trafalgar Square in support of the Unity March in Paris on January 11.

© Sputnik

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.

Hundreds attend Je Suis Charlie rally in Glasgow http://t.co/kZOF4br7mQ pic.twitter.com/u8erDpX1c3

— The Evening Times (@TheEveningTimes) January 11, 2015

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.

MI5 boss warns 'we cannot stop every UK terror threat' #security http://t.co/AjM43IRZ1V pic.twitter.com/6O7nbxl1IH

— Visual Security Ltd (@SecurityInUK) January 11, 2015