MISSISSAUGA, Ont. - The death of Poland's president and 95 others in a plane crash while en route to commemorate a Second World War massacre is a "cruel irony," Prime Minister Stephen Harper told thousands gathered for a remembrance mass Thursday.

Polish President Lech Kaczynski, his wife and the others -- including many high-ranking government and military officials -- died Saturday when the Tu-154 plane went down while trying to land in dense fog at Smolensk near Katyn.

The delegation was on its way to a commemoration of the 70th anniversary of the massacre of Polish officers at Katyn.

"The fact that this tragic event took place so near Katyn and the marking of the 70th anniversary of the massacre of thousands of Poland's military, political and intellectual leadership is a cruel irony indeed," Harper said at the outset of the mass.

"Few countries in the world have endured so much, so often at the hands of unforgiving fate as has Poland. But fewer still have demonstrated so much resilience in response to hardship."

The people of Poland can draw strength from the one million Canadians of Polish descent who have made immeasurable contributions to this country, Harper added.

"Whenever Poland grieves, Canada grieves," he said. "We stand with the victims of Katyn then and now."

The mass at St. Maximillian Kolbe Roman Catholic church, which drew thousands of Polish-Canadians, was led by Toronto Archbishop Thomas Collins.

Earlier this week Harper declared Thursday a national day of mourning for Canadians to stand in solidarity with the people of Poland and the Polish-Canadian community as they deal with the loss.

"It's a symbolic gesture but a very important one for Polish Canadians and all Canadians," said Rev. Pawel Ratajczak, director of Catholic Youth Studio.

"It's a day to mourn but also a day to be thankful."

Big screen televisions were set up in a hall next to the church for the thousands who would not be able to get inside, and mourners left dozens of candles burning in lanterns outside.

The mass was also attended by opposition leaders Jack Layton and Michael Ignatieff.

Harper and Russian President Dmitri Medvedev are among many world leaders planning to attend a state funeral for Kaczynski on Sunday at Krakow's 1,000-year-old Wawel Cathedral, the main burial site of Polish monarchs since the 14th century.