UPDATED 3.42pm: Police have issued arrest warrants for two men still at large following a terror attack in Paris.

12 people are dead after the attack on the offices of the Charlie Hebdo newspaper.

18-year-old Hamyd Mourad handed himself in after seeing his name circulating on social media.

Brothers Said and Cherif Kouachi are still on the run.

Police have described them as 'armed and dangerous'. 

Several people linked to the pair are reportedly also in custody.

Police operation underway

A police operation has been underway in the city of Rheims for the gunmen wanted over the murder of 12 people in Paris.

France's elite anti-terrorist unit is carrying out a raid in the northeastern city of Rheims as part of the hunt for the gunmen who attacked the satirical newspaper Charlie Hebdo.

A member of the unit says either the suspects will be able to escape, or "there will be a showdown".

Journalists at the scene have been urged to remain "vigilant".

Paris in virtual lockdown

Paris is in a virtual lockdown following the bloody attack on Charlie Hebdo.

Twelve people died and 11 were wounded when gunmen massacred staff at the satirical newspaper.

Troops are now at railway stations, armed police have been posed outside media buildings, and there is ultra-tight security at department stores:

Parisians have responded by taking to the streets in their tens of thousands to show their support for the victims.

French author Bernard-Henri Levy says free speech must be defended.

"The task for any French writer, any French journalist, is to help the spirit of Charlie Hebdo, which is the spirit of France, to survive.

National day of mourning

French President Francois Hollande has declared a national day of mourning tomorrow.

Mr Hollande says the country will do everything it can to track down the killers.

"Tomorrow will be a day of national mourning, I have decreed this.

"At 12pm there will be a moment of contemplation in all public services and I invite all people to join.

"Our own best weapon is our unity of all our citizens faced with this trial, this hardship.

"Nothing can divide us, nothing can and must oppose us, once against the other - nothing can separate us."

Graphic videos appear online

The execution of a police officer is among the horrific scenes captured in a string of videos posted online in the aftermath of the Paris newspaper massacre.

Clips recorded by eye-witnesses on smartphones showed two masked gunmen dressed in black firing what appear to be rifles as terrified onlookers fled.

One video circulated on Facebook shows the two gunmen stood next to a Black Citroen car firing at distance at a police officer stood on the pavement on a tree-lined boulevard.

The camera then pans to show the officer, now wounded, on the floor clutching his leg and raising his arms in an apparent gesture of surrender.

The assailants march towards him before one shoots him casually in the head at close range.

In footage filmed from a nearby rooftop one of the attackers can be heard shouting "Allahu Akbar" - meaning God is the greatest.

Muslims under threat?

The attacks may not do anything to help Muslims living in France.

Strategic analyst Paul Buchanan says its more likely the attacks will affect their ability to live peacefully in the country.

"The Muslim comm in France is going to suffer some backlash because there's already very strong national resentment in France against the Muslim community."

Nina Ognianova from the Committee to Protect Journalists has condemned the attack.

"This horrific event in Paris today shows that the threat to journalism and to freedom of expression and freedom of speech is indeed a global threat."

Paris' deputy mayor Patrick Klugman told CNN the city will not be scared into submission by terrorists.

"We must take all the measures to protect the population, to higher the level of protection, but also to continue to live.

"If not the terrorists will win the battle and this we cannot accept."

Author Salman Rushdie - one of the first people to feel the weight of religious terrorism - has denounced the "religious totalitarianism" of the attack.

He says religion, which he describes as a medieval form of unreason, when combined with modern weaponry becomes a real threat to our freedoms.

Rushdie says we must all defend the art of satire, which has always been a force for liberty against tyranny, dishonesty and stupidity.

World leaders condemn attack

US President Barack Obama has condemned the attack and directed his administration to provide any assistance needed to help bring the terrorists to justice.

US Secretary of State John Kerry has given an address on the attacks, saying America stands with France in anger and outrage.

"We stand with you in solidarity and in commitment, both to the cause of confronting extremism and in the case of which the extremists fear so much and what has always united our two countries: freedom."

UN Secretary-General Ban Ki Moon has called it a horrendous and cold-blooded crime.

"It was also a direct assault on a cornerstone of democracy - on the media and the freedom of expression."

British Prime Minister David Cameron says he's spoken to his French counterpart and offered his full support.

"And we stand absolutely united with the French people against terrorism and against this threat to our values: free speech, the rule of the law, democracy and it's absolutely essential that we defend those values today and everyday."

German Chancellor Angela Merkel is calling it a tragic day.

She's made a televised address at Downing Street, alongside UK Prime Minister David Cameron.

Her words are translated.

"I think it was a very moving moment when we were able to address, both of us, Francois Hollande on the phone. We assured him that we would do everything we can to help him in this very desperate hour, we stand by the French people."

John Key has issued a statement saying the targeting of journalists going about their daily work is an attack on the fourth estate and the democratic principle of freedom of speech.

He says French authorities have confirmed the shooting was politically motivated and he hopes those responsible will be brought to justice.

At this stage there are no reports of any New Zealanders caught up in the attack.

The Queen has released a statement expressing sincere condolences to the families of those who have been killed.

She says she and Prince Phillip are sending thoughts and prayers to all those who have been affected.