TOULON, France : President Francois Hollande on Friday paid tribute to the hundreds of thousands of troops who, 70 years ago in World War II, launched the southern invasion of occupied France that opened up a second western front against Hitler's Nazis.

Joining Hollande for Friday's ceremonies were 15 leaders from France's former African colonies, in recognition of the key role soldiers from these countries played in liberating France, two months after D-Day smashed the first hole in Hitler's defences. Around 240 veterans, many in their 90s, also took part in the commemorations, including around 40 from France's former colonies.

"By their sacrifice, these men bound our country to Africa with a bond of blood that no one can undo," said Hollande, as veterans applauded, their medals glittering in the southern French sun.

The southern landings "changed everything" said Hollande. "It was here that France liberated itself, with the support of her Allies."

After the success of the Normandy beach landings, the Allies needed to open up a second front in France to squeeze the demoralised German army and retake the ports of Marseille and Toulon to resupply forces.

The result was "Operation Dragoon", launched on the beaches near Marseille on August 15, 1944, with a total force of 450,000 men.

In contrast with the Normandy landings, where there was only a token French army presence, more than half (250,000) of the invading force was French.

In turn, this French force had a large number of troops from France's then colonies, mainly from Algeria and Morocco, but also infantry from Senegal and soldiers from Pacific islands.

The invasion "succeeded much more quickly than expected," historian Jean-Marie Guillon told AFP.

The operation was "inextricably linked" to the better-known Normandy landings on June 6, he said.

"They were supposed to happen at the same time. It was only in April 1944 that they were separated, for practical reasons: there weren't enough boats."

Facing the Allies was the German 19th army with 250,000 badly equipped and shattered troops spread all along the coast, poorly defended with barbed wire, mines and heavy artillery.

As a result of this mismatch, the bloodshed seen on the Normandy beaches was largely avoided as the Germans quickly realised they could not defend their position.

On the evening of August 15, of the 100,000 men who had successfully landed, around 1,000 had fallen, death on a much smaller scale than D-Day, which saw some 10,000 casualties.

"We underestimate the importance of these landings," said Guillon, adding that they were less well-known than the D-Day invasion because "they went too well!"

The Germans retreated rapidly, chased into the mountains by the rampant Allies, who were able to establish a supply base for the later invasion of Germany itself.

Hitler described it as "the saddest day of his life," and it led 10 days later to the joyful liberation of Paris.

Unlike ceremonies held in Normandy to mark the 70th anniversary of D-Day on June 6 - attended by a phalanx of world leaders including US President Barack Obama and Queen Elizabeth II - Friday's events were more low key.

In addition to the African leaders, lower-level representatives from the 13 other countries that took part in the landings will attend an international ceremony on the Charles de Gaulle aircraft carrier, anchored off Toulon.

There will also be a naval parade involving a dozen ships from former Allied powers and organisers expect hundreds of thousands of people to take part in the celebrations that extend into the weekend.

No bilateral diplomatic meetings were expected on Friday but international crises will undoubtedly be addressed at a gala dinner of the leaders on the Charles de Gaulle.