Update: Be prepared for British casualties in Paris attacks warns Prime Minister

And he warned people to be prepared for Britons to be among those who died in the atrocity.

It is understood that 129 people have been killed and more than 350 wounded in the attacks in the French capital.

There was a police operation at the Bataclan concert hall where hostages were taken and further attacks across Paris.

Nick Alexander, from the UK, died in the attack at the concert hall.

Witney MP Mr Cameron tweeted: "I am shocked by events in Paris tonight. Our thoughts and prayers are with the French people. We will do whatever we can to help."

Oxford East MP Andrew Smith told the Oxford Mail people would stand with France against the attackers.

He said: "It is an appalling attack and my thoughts go to those people killed and injured and their families.

"I know that everyone in Oxford will feel great sorrow and solidarity for the people of Paris and France after such an awful attack.

"I have got French friends in Oxford and I know how this touches everybody.

"Regardless of nationality we will feel a threat from this awful IS organisation and its perverted ideas.

"I am sure that in churches, mosques and synagogues people's thoughts and prayers will be for the families affected and also for all our communities which must stand against this evil."

Oxford West and Abingdon MP Nicola Blackwood added: "We stand with the people of #Paris tonight. Thoughts and prayers with victims and loved ones. Strength to emergency responders."

And Henley MP John Howell tweeted: "Our thoughts and prayers tonight with Parisians for the horror they have faced."

An explosion was reported near the Stade de France where France's football team played Germany, watched by French President Francois Hollande.

He declared a state of emergency.

Celebrity chef Raymond Blanc, who opened his first Brasserie Blanc restaurant in Oxford and runs hotel Le Manoir aux Quat'Saisons in Great Milton, tweeted: "I just heard that my France was stricken again. I hear some terrible news. I do not understand. Why so much hate.

"What a deep sadness all across France .So many innocent people died.

"Thank you to all my British friends who gave their love and compassion to French families for the terrible loss of their loved one."

The French president has denounced the terror attacks that killed at least 127 people in Paris as an "act of war" and blamed the Islamic State terror group for the carnage.

Speaking after an emergency meeting of senior government and security officials at the Elysee Palace, Mr Hollande declared three days of national mourning and vowed that France would be "pitiless" in its response to terrorism.

Not-for-profit organisation the Alliance Francais D'Oxford has vowed to keep fighting the good fight.

Director Florence Rossignol, whose family hails from Normandy near Paris and Mougins in the south, said: "I was watching everything on TV. It's shocking and dispiriting and there is no justification for such acts.

"My father, brothers, mother, stepfathers and cousins are in France. We have some French family staying here at the moment but I think they will have trouble going back. It's awful to think that this has happened again soon after the Charlie Hebdo attack.

"My job is here, helping people to enjoy French culture here in Oxford. We are going to continue with the events that we planned."

A number of Britons are feared dead following the terror atrocities in Paris, David Cameron has said.

The Prime Minister condemned the "brutal and callous murderers" behind the series of attacks in the French capital.

"We must be prepared for a number of British casualties," he warned.

Speaking in Downing Street he said the terror threat level in the UK would remain at "severe", but the attack would prompt a review of plans and suggested the threat posed by Islamic State was "evolving".

In a message of solidarity to the people of France he said: "Your values are our values, your pain is our pain, your fight is our fight."

The Prime Minister added: "The events in Paris are the worst acts of violence in France since the Second World War, the worst terrorist attack in Europe for a decade, a horrifying and sickening attack.

"Our hearts go out to the French people and to all those who lost loved ones.

"Today the British and French peoples stand together as we have so often before in our history when confronted by evil."

Mr Cameron said the full picture of the attacks was still emerging but "we must be prepared for a number of British casualties and we are doing all we can to help those caught up in the attack".

He said: "These were innocent victims enjoying a Friday night out with friends and family, no doubt at the end of a hard week. They were not seeking to harm anyone, they were simply going about their way of life - our way of life.

"They were killed and injured by brutal and callous murderers who want to destroy everything our two countries stand for: peace, tolerance, liberty. But we will not let them.

"We will redouble our efforts to wipe out this poisonous, extremist ideology."

A manhunt is continuing  for accomplices of the gunmen who targeted the concert hall and the French national football stadium and sprayed the terraces of bars and restaurants with gunfire in at least six almost simultaneous attacks.

French authorities said they believed all eight of those involved in the attacks were dead - seven of them killed by suicide bombs - but Paris's chief prosecutor said it was possible other terrorists were still on the run.

Policing was being strengthened at ports and major events in the UK, and Mr Cameron is chairing a meeting of the Government's Cobra emergency committee which could raise the official assessment of the threat from international terrorism from its current "severe" level.

Imam Monawar Hussain, Founder of The Oxford Foundation, said following the attacks: "We are deeply shocked and saddened to witness the appalling acts of violence perpetrated against innocent civilians enjoying an evening out with their friends and families in Paris."