Mr Leonard was attending a service at Summerlee Industrial Museum in Coatbridge to remember all those killed in the workplace.
Among them are the 19 miners who died when the nearby Stanrigg Colliery collapsed 100 years ago this July.
Mr Leonard said: “We meet to commemorate their memory and the memory of all of those others since who have lost their lives at work.
“We remember the dead and fight for the living. So we also meet to renew our determination and our faith that there is another, better way.
“We need better laws, stronger trade union rights and political as well as industrial organisation.
“Because we must always remember, that this great movement of ours was not founded simply to fight for the weekly wage.”
Mr Leonard used his address to call for an end to the UK opt-out of the European Working Time Directive upon Brexit.
The directive prevents people from working more than 48 hours a week on average but UK workers can choose to “opt out” of this cap.
The Scottish Labour leader said more than 250,000 Scots work more than 48 hours, with many employed in agriculture and the construction industry.
“It is no coincidence that these are the two sectors which have the worst health and safety records and the highest number of workplace fatalities in Scotland,” he said.
“We need an end to the long hours, low productivity, low wage economy starting with an end to the UK opt-out as part of a managed reduction in working time with no loss of earnings.”