The first minister has warned people to be prepared for the possibility that there might be Welsh victims of the "horrific" Paris attacks.
Carwyn Jones was part of the emergency Cobra meeting convened by David Cameron on Saturday morning after nearly 130 people were killed in the French city.
To show solidarity, the Senedd will be lit up in the colours of the tricolour on Saturday and Sunday nights.
It comes as Welsh people in Paris have described their panic and fear.
France's President Francois Hollande said the near-simultaneous attacks in Paris, which also left at least 180 people wounded, were an "act of war" organised by the Islamic State (IS) militant group.
Eight gunmen and suicide bombers struck at bars, restaurants and a concert hall on Friday evening.
Mr Jones said he urged members of the public to be "vigilant" and to report anything suspicious to the police.
"I was part of the emergency Cobra meeting," he told BBC Wales.
"The first thing is that we are not clear if there are casualties from Britain and we should prepare ourselves for the possibility that there may be casualties from Britain and indeed from Wales.
"It's too early to tell yet if that is the case, but perhaps we should be ready for that."
He added that flags would be flying at half mast at all Welsh government buildings, while a planned reception for the Wales football team, due to be hosted by the first minister, was cancelled as a mark of respect for the victims.
The Muslim Council of Wales condemned the attacks, saying it rejected IS's attempt to" justify their violence through religion".
"Muslims globally, and here in Wales, reject the so called Islamic States' political aspirations in Syria and Iraq," secretary general Saleem Kidwai said.
"We mourn the deaths and offer our thoughts and prayers to the families who have lost their loved ones."
The worst bloodshed was at the Bataclan hall in the east of the city, where gunmen took hostages at a sold-out rock concert featuring band the Eagles of Death Metal.
It was part of the Les Inrocks musical festival, which Merthyr Tydfil band Pretty Vicious were also performing in.
Their gig was at a nearby venue, with members later tweeting to say they were "safe and sound" despite events in other parts of the city.
Anthony Pickles, from Penarth, who was at a restaurant near the concert hall, said: "There was a sense of panic on the streets... I think today will be a very, very sombre day in Paris."
Jonathan Hill from Cardiff, who was also close to the concert hall, described seeing a "Good Samaritan" standing in the street and shouting to people to get indoors.
"I heard three distinct firing shots," he said. "I saw someone collapse to the floor."
Seiriol Hughes from Caernarfon was at the Stade de France watching the France versus Germany match when fans heard explosions near the ground.
English teacher Mr Hughes described hearing a "loud bang, the noise of an explosion which silenced the crowd" in the stadium which people thought to be a firework.
He said only after the game, as the crowds left the stadium, did people begin to get a sense of what had happened.
"Something must have triggered and people just started running away, picking up their kids and running and with a look of panic and fear in their eyes and we couldn't work out why," he said.
"Eventually, when we were getting a train away from the stadium we began to get phone signal and then messages started to come through that something had obviously been going on and those explosions were not fireworks."
Teacher Ceri Davies from Penarth was in a pub nearby which pulled down its shutters to keep customers safe.
"We are in a state of shock," he said. "We were advised to stay in the pub so they closed the metal barriers."
Welsh rugby referee Nigel Owens was in the south of France, where he had been due to take charge of the European Rugby Champions Cup match between Toulon and Bath.
But that has been cancelled in the wake of the attacks.
He tweeted: "My thoughts are with all the French people who have lost loved ones yesterday. The world is becoming a very sad and dangerous place."
Welsh secretary Stephen Crabb, whose wife is French, tweeted: "Wales stands as one with the people of France today."