After 100 years, a bright and colourful celebration will mark the centenary of the Pont de Québec in Quebec City and highlight the history of what was once the world's longest bridge.

​The Pont de Québec is one of the city's icons. The riveted steel truss structure was built in 1919 and it's also a National Historic Site of Canada.

While entertainers, musicians and fireworks are part of today's festivities, a much more somber ceremony was held earlier this week.

Those at the commemorative mass on Sept. 20, the bridge's official birthday, gathered to recognize the 89 workers who died while building it.

Pont de Quebec

A view of the Pont de Québec as the sun rises in winter 2015. (Geneviève Poulin/Radio-Canada)

'You could hear their cries'

The Pont de Québec, which links Quebec City to Lévis, was the largest bridge in the world for 12 years after it was built. It still remains the world's largest cantilever bridge.

But the innovation of the rail, road and pedestrian bridge came at a terrible price.

The entire southern half of the bridge collapsed while it was still under construction on Aug. 29, 1907. 

While about 100 people were working on it, 15,000 tonnes of iron fell into the St. Lawrence River in a matter of seconds. The bridge could not support its own weight due to a structural flaw.

Seventy-seven of the workers died.

The tide of the St.Lawrence was low at the time of the collapse. Some workers were still alive when they were caught in the crumpled metal, said Michel L'Hébreux, who authored Le Pont de Québec, a book on the history of the bridge.

Pont de Quebec

The Quebec City bridge collapsed twice during its construction. (Radio-Canada archives)

"To get some of them out, they even cut off some people's arms and legs," he said. "For some of them, their bodies were caught. You could hear their cries, wailing with suffering."

Several workers also died from drowning when the tide came in, L'Hébreux said. Only 35 bodies could be located after the tragedy — less than half of the total death toll.

"Forty-one of them were left to decompose at the bottom of the river. They were never found," said L'Hébreux.

Mohawk iron workers

Of those who died in the first collapse, 33 of them were ironworkers from Mohawk territory of Kahnawake.

"That was half the population of the men at that time, the workforce," said Chief Martin Laborgne, who attended Wednesday's commemorative mass and First Nation ceremony for those who died.

Martin Leborgne

Martin Leborgne, a chief from Kahnawake, says the fatal Pont de Québec collapse still impacts the community today. (Radio-Canada)

He said it had a profound and lasting effect on the people who live there.

"Since then, the women in the community said that from that day on, we would never put so many men on one job because of the risks," he said. "And because we didn't want to relive another loss like that one."

A second collapse

After the first collapse, the project nonetheless continued.

With the construction of the railway on the south shore, the city was having trouble competing economically without one at the time.

In 1916, a second fatal collapse rocked the bridge.

Pont de Québec 1916

A McCord Museum photo of the collapse of centre span of the Pont de Québec in Quebec City, 1916 (Courtesy of the McCord Museum)

A completed central section fell while it was being lifted into place in front of a crowd of 100,000 people who had assembled especially for the occasion.

Thirteen people died in front of the eyes of Lomer Gouin, the Quebec premier at the time, and Wilfrid Laurier, the federal opposition leader and former prime minister, who were part of the crowd.

With files from Radio-Canada's Maxime Bilodeau, Cathy Senay and CBC's Angelica Montgomery