A huge security operation will be deployed at unity marches in France today, after it emerged that a young woman linked to the Paris massacres may have escaped security forces' clutches and fled to Syria.

Hayat Boumeddiene - the partner of terrorist Amedy Coulibaly- may have left the country before the crisis reached a bloody conclusion, reports suggest.

Last night Turkish authorities reportedly disclosed that they believe the 26-year-old - France's most wanted woman -may be in Syria after flying to Istanbul more than a week ago, before vanishing near the border between the two countries.

Meanwhile Britain's Sunday Times is reporting that Britain is on the brink of raising the terrorist threat level to critical - the highest category.

Intelligence officers are working to establish the background to the synchronised strikes which terrorised the French capital and claimed the lives of 17 victims over three days.

Coulibaly and co-conspirators Said and Cherif Kouachi, the brothers who killed 12 people at the offices of satirical magazine Charlie Hebdo on Wednesday, were finally killed during simultaneous police assaults in a dramatic conclusion to France's deadliest terror attack since 1961.

Thousands of police and military personnel have been drafted in to secure the Paris rally, as the country remains on high alert.

Prime Minister Joseph Muscat will join leaders from across the globe in a show of unity. Up to a million people are expected to turn out in the capital.

French interior minister Bernard Cazeneuve said a total of 1,350 military personnel and 2,000 police will be on duty at the rally, which is expected to draw crowds of up to 1 million.

Sharpshooters will be positioned on roofs while drains will be scoured in advance of the event, he said.

Mr Cazeneuve said: "There will be a public order plan of exceptional magnitude to make sure that the rally goes well and also to guarantee maximum security.

"French people will express as a mass their attachment to the values of Republican freedoms and freedom of expression after the bloody attacks that the terrorists inflicted."

Around 50 heads of state and government are expected to gather with French President Francois Hollande at the Elysee Palace before embarking on what is billed an unprecedented show of international solidarity near the head of the march through the capital.

Among those in a phalanx of senior figures marching behind the families of victims are expected to be Spanish prime minister Mariano Rajoy, German chancellor Angela Merkel, Italian prime minister Matteo Renzi, Ukrainian president Petro Poroshenko - with Russia also represented by foreign minister Sergei Lavrov.

Also attending are Palestinian president Mahmoud Abbas and Israeli premier Benjamin Netanyahu - who has directly encouraged French and other European Jews to flee the threat of terror and move to Israel.

Mr Hollande is due to meet leaders of the Jewish community before the march.

The US will be represented by attorney general Eric Holder who is in the city along with other security ministers for talks about the threat posed by Islamist extremism.

With up to a million people expected to join the march, public transport has been made free all day to encourage people to attend without cars.

And the now-familiar slogan of solidarity "Je Suis Charlie" - or "I Am Charlie" after the name of the satirical magazine - is to be seen everywhere, from bus shelter posters on the route to the corner of the screens of public television stations.

Scrutiny has fallen on how closely the Kouachi brothers were monitored and last night there were claims that surveillance of them was stopped six months before the massacre because they were deemed low risk.

Prosecutors have said there were 500 calls made between the couple and the Kouachis, although it is has not been confirmed what period the contact took place over.

Meanwhile, amid the mourning, stories of heroism have emerged.

The brothers were eventually cornered at the printing works about 30 miles outside Paris in Dammartin-en-Goele, where they took one man hostage.

But unknown to the terrorists, another man - Lilian Lepere - was hiding underneath a sink in the building, helping direct the police to the killers using his mobile phone.

His boss Michel Catalano said he expected to be dead by Friday night, and told how he was "terrified" throughout the ordeal that the brothers would discover the hidden worker.

He said: "I could immediately see there was a situation of danger. I told my employee to hide. I knew two of us couldn't hide.

"At that point I thought that was the end. They came in, they weren't aggressive. They said 'don't worry, we just want to come in'."

He offered the intruders a drink and made coffee for them before one of his suppliers arrived at around 9am.

"I didn't know where Lilian was hidden. I knew he was hidden but I had no idea where. I didn't want them to go to the end of the building.

"I must admit that in fact they (the brothers) weren't aggressive as far as I was concerned.

"I didn't get the impression they would harm me, as unbelievable as it sounds. Perhaps they had an ounce of humanity because they let me out."

Lassana Bathily was hailed a hero for his role in hiding hostages as they sought refuge from Coulibaly at the Hyper Cacher supermarket in Porte de Vincennes.

Mr Bathily, originally from Mali in west Africa, is said to have shepherded terrified customers to safety in a chiller as the Islamic gunman - the main suspect in the fatal shooting of a policewoman a day earlier in the Parisian suburb of Montrouge - murdered four of the 19 hostages before police stormed the building.

They have been named as Yoav Hattab, Philippe Braham, Yohan Cohen and Francois-Michel Saada.

Armed officers were able to hear what was happening inside through a telephone left off the hook.

Extracts of Coulibaly ranting at hostages in the store emerged. According to RTL, Coulibaly railed against the French state and mocked his hostages for paying taxes.

Meanwhile Malik Merabet - whose police officer brother Ahmed Merabet was killed by the Kouachis on Wednesday - pleaded for unity, saying Islam must not be conflated with extremism.

During an emotional news conference he broke down when asked about graphic images of his brother lying injured on a pavement before one of the gunmen shot him dead at close range.

A range of London landmarks will today have an image of the Tricolore beamed on to them in a show of unity with the French.