David Cameron has joined French president Francois Hollande at the scene of one of the Paris atrocities as a major anti-terror operation continued in neighbouring Belgium.
The Prime Minister visited the Bataclan concert hall, where scores of people were killed in the massacre by Islamic State (IS) gunmen.
The two leaders viewed the floral tributes placed outside the music venue, where fans of rock group Eagles Of Death Metal were gunned down.
Mr Cameron said the two men stood "shoulder to shoulder" and he paid tribute to the "courage of the French people" following the marauding attack on their capital.
Meanwhile Belgian police arrested 16 people in a major anti-terror operation, but on-the-run Paris attacker Salah Abdeslam remains at large, a federal prosecutor has said.
No arms or explosives were found in 19 raids across Brussels - including the Molenbeek area from where the Paris attack was planned - or two in Charleroi, they said.
Abdeslam was said by Le Soir newspaper to have been identified fleeing in the direction of Germany in a BMW.
The authorities urged a social media blackout of operations as the desperate manhunt continued for surviving members of the group behind the gun and bomb murders of 130 people in the French capital.
Brussels remained on high alert over fears of a Paris-style IS attack.
Schools, universities and the underground system will remain closed and some workers have been advised to stay at home while key suspects remain at large.
As the search for the jihadists went on, world leaders were contemplating how to respond to a string of outrages - also including the downing of a Russian airline over Egypt - with the UK moving nearer to joining allied air strikes in Syria.
Mr Cameron will present the case for escalating British military involvement to Parliament later this week - with the Paris attacks and a unanimous United Nations Security Council resolution apparently galvanising support among MPs.
His talks with Mr Hollande in Paris come as the French president starts an intensive push to create a "grand coalition" to destroy IS - also known as Isil, Isis and Daesh - which will take him to Washington and Moscow in the coming days.
A Commons vote on air strikes could be held as early as next week and Chancellor George Osborne said the deaths of 130 on the streets of Paris and the UN resolution backing "all necessary measures" were swaying the argument.
"We'll make the case as a government, we will allow MPs to digest that response and then we will see where we stand.
"Frankly, Britain has never been a country that stands on the sidelines and relies on others to defend us," he said.