Six Canadian soldiers serving with the Nato-led force in south Afghanistan have been killed, while another Nato soldier died in a separate incident.
Fifty-one Canadian soldiers have died in Afghanistan so far
The Canadians were killed when the vehicle they were travelling in hit an explosive device, Nato's International Security Assistance Force (Isaf) said.
"Our hearts ache for them and their families," said Canadian Prime Minister Stephen Harper.
It is the worst single incident for the Nato-led force since 2005.
Mr Harper confirmed the deaths in the town of Verlinghem, France, while on a visit to mark the 90th anniversary of the World War I Battle of Vimy Ridge.
"Sadly, today has been a difficult day in Afghanistan. We have learned that an incident has claimed the lives of six Canadian soldiers and injured a number of others," he said.
"I know that as we gather here on Easter Sunday our hearts and prayers are with them."
At least one soldier was also injured in the explosion, which caused the most casualties suffered by Canadian forces in a single day since they deployed to Afghanistan in 2002.
Addressing a group of veterans, Mr Harper drew parallels with WWI and the challenges faced by troops in Afghanistan today.
"For these men and women, the terrain of Kandahar province looks as desolate and dangerous as Flanders field did 90 years ago," he said.
Elsewhere in southern Afghanistan, another roadside blast killed one Nato soldier and wounded two others earlier in the day, Nato said.
It did not release the nationality of those soldiers.
An Isaf spokeswoman told the BBC there had been no civilian casualties in the blast that killed the Canadians and that all signs pointed to the Taleban.
"Certainly it lends itself to the type of tactic that Taleban extremists use," Lt Col Angela Billings said.
"Because they cannot beat us conventionally or tactically, they resort to this type of tactic in order to hide in the shadows."
Canadian and British troops make up most of the Isaf forces in the south.
The Taleban are maintaining strong opposition to Nato, particularly in the south and east.
Last year saw the worst fighting in Afghanistan since coalition troops ousted the Taleban in 2001 with some 4,000 people believed to have been killed - about a quarter of them civilians.
Helmand has been the focus of a recent operation by Isaf troops against militants.
About 100 insurgents have been killed in it so far, officials say.