Up to a million people – including dozens of world leaders – will converge on Paris on Sunday in a show of defiance and unity after terrorist attacks that left 17 people dead last week in the French capital.

In what will be one of the most significant and emotional public gatherings in postwar French history, President François Hollande will be joined by David Cameron, Angela Merkel and the Italian prime minister, Matteo Renzi, in an unprecedented solidarity march through the centre of the city.

The French capital has been in shock since the massacre of 12 people at the offices of the satirical magazine Charlie Hebdo by two French-born brothers who claimed to be avenging the prophet Muhammad. The horror and fear were amplified when another attacker gunned down a traffic policewoman on Thursday and returned to shoot four shoppers at a Jewish supermarket on Friday. All three attackers were killed in shootouts on Friday. The French prime minister, Manuel Valls, said Sunday’s demonstration would be “a cry for freedom” and a reassertion of “the values of 1789 … It will be an unusual demonstration that will no doubt go down in history. It will show the strength and dignity of the French people, who will shout their love for liberty and tolerance. Come in numbers.”

Amid a swirl of hostile rhetoric, including from media mogul Rupert Murdoch who said on Twitter that all Muslims should be held responsible for the Paris attacks, the family of the Muslim policeman gunned down outside the Charlie Hebdo office spoke out for the first time. Ahmed Merabet was proud of his job and of the country he died defending, they said. “I address this to all the racists, the Islamophobes and the antisemites: you mustn’t mix up extremism with Muslims,” said Ahmed’s brother, Malik Merabet. “The madmen have no colour nor religion. Islam is a religion of peace, of love … my brother was a Muslim and he was killed by two terrorists, by false Muslims.”

Six of the supermarket hostages, including a toddler and his father, were saved by a Muslim store employee, Lassana Bathily, who hid them and then risked his life to conceal their presence.

“When they came running down [into the basement] I opened the door of the fridge,” he told French channel BFM TV. “Several came in with me. I turned off the light and the fridge. I closed the door and I said ‘You stay quiet there – I’m going back out’.” The four hostages killed in the attack on the Jewish supermarket were named as Yoav Hattab, Philippe Braham, Yohan Cohen and François-Michel Saada. The Conseil Représentatif des Institutions Juives de France, the main French Jewish umbrella organisation, condemned the antisemitic nature of the attack, saying: “These French fellow citizens were slaughtered coldly and pitilessly, because they were Jewish.”

Travel on public transport in and around Paris will be free on Sunday to allow hundreds of thousands to get to and from the demonstration, and at least one train operator has cut prices for people coming from other cities. Around 250,000 people had already joined marches in other French cities on Saturday.

The government has promised a massive security operation to keep marchers safe amid fear of another attack. Police are hunting Hayat Boumeddiene, the 26-year-old partner of supermarket killer Amedy Coulibaly, who they say may have been an accomplice in Thursday’s shooting. On the run now, she was close to extremist friends of her husband, and could be armed and dangerous. But there were reports that Boumeddiene had flown to Turkey on 2 January and was in Syria by the end of last week.

“There will be a public order plan of exceptional magnitude to make sure the rally goes well and to guarantee maximum security,” said French interior minister Bernard Cazeneuve. “The terror alert will be maintained at its highest level in the Ile-de-France.”

At least 500 extra military personnel will be on duty in the greater Paris region. Thousands of uniformed and plain-clothes police stationed in and around the march will be looking out for potential attackers trying to drive into marchers, or taking up positions on rooftops or balconies, a police source told the AFP news agency.

Cameron will fly out to Paris on Sunday to attend the march, and London landmarks including Tower Bridge and Trafalgar Square will show the colours of the French flag from 4pm. The attacks in France will be high on the agenda when Cameron visits Washington for talks with President Barack Obama on Thursday and Friday.

A “survivors’ issue” of Charlie Hebdo will be published on Wednesday and will be sold outside France because of the huge global attention – marking a turnaround for a publication that just a week ago seemed on the brink of folding. One million copies will be printed instead of the usual 60,000.

The French government, newspapers and companies around the world have pitched in cash, transportation and other help to ensure the issue reaches an unprecedented audience. All money from the sales will go to the families of the 12 people murdered in the attack on Wednesday.

Keeping Charlie Hebdo in print is being seen in France as an act of defiance against the Islamists who sought to extinguish it, and as a statement in support of free speech. However, the attacks have also emboldened militants and their sympathisers, the Associated Press reported, with supporters of jihadist groups on Twitter and other social media lionising the Kouachi brothers. They have been organising under the Arabic hashtags #Parisattack and #Parisisburning, with some calling the newspaper assault a holy attack.