The famous Paris landmark switched off its lights as the clock struck midnight, hours after the UK suffered its worst attack since 2005.
Five people were killed and about 40 injured after a car ploughed into pedestrians and a suspected Islamist-inspired attacker stabbed a policeman close to Britain's parliament.
The dead, in what police called a "marauding terrorist attack," included the assailant and the policeman he stabbed - named last night as PC Keith Palmer.
Paris stands in solidarity with London and the UK
The famous landmark went dark to mourn the victims of the terror attack
PC Keith Palmer died after being stabbed by a terror suspect
The other three victims were among those hit by the car as it sped across Westminster Bridge before crashing into railings just outside parliament.
Paris joined cities around the world are showing solidarity with London.
Tel Aviv City Hall was illuminated in the colours of the Union flag to pay homage to those killed and injured.
The Union flag was projected in lights in Tel Aviv
Paris Mayor Anne Hidalgo earlier expressed her "solidarity" in a message to her London counterpart Sadiq Khan, adding: "Paris and London have very close links which have become stronger in the last few decades."
She said the two capitals "share a common love of freedom and an attachment to democracy" and were "tolerant and cosmopolitan cities, open to the world".
People from all over the world are taking to social media with the hashtag #PrayforLondon.
The lights of the Eiffel Tower will be turned off, shrouding Paris in darkness, at midnight
Eyewitnesses described how the attacker deliberately mounted the pavement with a car on Westminster Bridge before hitting more than 12 people - including three police officers returning from a commendation ceremony.
The driver then reportedly got out of his car at the Houses of Parliament where he attempted to force his way past a security check-point.
After being confronted by police he reportedly attacked an officer with a knife before he was shot a number of times.
London was in a state of chaos after the attackFour people died and dozens were injured after the terror attack at Westminster
A dramatic picture from the scene shows the suspected terrorist being treated by medics as an armed police officer points a gun at him.
As the attack took place, Prime Minister Theresa May was bundled from the House of Commons by police and Parliament was placed under lockdown.
Big Ben has been illuminated in red in tribute to the victims of the attack.