Emmanuel Macron has described a bloody police crackdown on Algerian anti-war demonstrators 60 years ago that led to many deaths as an “unforgivable” crime.

Attending a memorial for those killed, Macron laid flowers at a bridge over the River Seine which marked a starting point for the protests in October 1961 that led to one of the darkest chapters of French postwar history.

He is the first French president to officially recognise that the “crimes committed that night … are unforgivable for the republic”, though he made no official speech. A statement issued by the Elysee admitted “the repression was brutal, violent and bloody”, but stopped short of an apology.

The events of the night of 17 October 1961 have never been legally investigated and are still shrouded in official obfuscation. Even today, the death toll from the police attack is unknown and disputed.

According to officials at the time, less than a handful of protesters died at the hands of Paris police, then led by the former Nazi collaborator Maurice Papon. Historians say the number killed, many of them beaten and thrown into the River Seine, was between 50 and 120, while Algeria has said the death toll in the “massacre” could have been as high as 300.

The Elysee statement acknowledged: “In addition to many wounded, several dozen were killed, their bodies thrown into the Seine. Many families never found the remains of their loved ones who disappeared that night. The president of the republic pays tribute to the memory of all the victims.”

The tragedy happened at the height of the Franco-Algerian war of independence when tens of thousands of Algerian protesters gathered at city landmarks to demonstrate against a “racist and discriminatory” curfew the authorities had imposed on them.

The protests were organised by the Paris wing of the Algerian National Liberation Front (FLN) that was fighting for the country’s independence and that had been accused of attacks on Paris police that left a dozen dead. The peaceful demonstrations, whose 30,000 participants included women and children, turned violent when Papon ordered a crackdown.

Officers shot at protesters and arrested up to 12,000 many of whom were herded on to buses and taken to makeshift detention centres, where they claimed police beat and tortured them and deprived them of food for days. Police also herded the panicked crowds on to city bridges over the Seine and were accused of throwing protesters into the river.

The official death toll was given as two and was only revised upwards when bodies began washing up on the riverbanks and turned up in woods near the detention centres.

In 2011 Macron’s predecessor, Francois Hollande, then the Socialist party’s presidential candidate, attended a 50th anniversary commemoration and unveiled a plaque at the bridge in Clichy, the Paris suburb where many of the victims lived.

Papon, awarded the Legion d’honneur by President Charles de Gaulle the same year as the Algerian killings, went on to hold a number of senior official posts. In 1998 he was convicted of crimes against humanity for his part in the wartime deportation of 1,690 Jews from Bordeaux where he was a police official, and stripped of his decorations and titles.

The Algerian war, which started in 1954, ended in Algeria’s independence from France in 1962.

The Elysee statement said France “recognises clearly established responsibilities”. It added that the Algerian war and “its trail of crimes committed on all sides” had bruised those involved “in their flesh and soul”.

France and Algeria are currently involved in a diplomatic row after the French government’s announcement it was substantially cutting the number of visas granted to visitors from its former north African colonies Algeria, Morocco and Tunisia, where many people have strong family ties in France. Algeria accused Macron of making tough gestures on immigration to court rightwing voters in the run-up to next year’s presidential election.