A man kneels at a makeshift memorial in Toronto to the late Polish President Lech Kaczynski and the other 95 people killed in a weekend plane crash in Russia. (CBC)

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 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, Russia, near Katyn.

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

Harper joined thousands of mourners at the mass memorial held at St. Maximilian Kolbe Parish in Mississauga, Ont., on Thursday.

"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."

More than 5,000 mourners attend

The mass at St. Maximillian Kolbe Roman Catholic church was led by Toronto Archbishop Thomas Collins. It was also attended by Liberal Leader Michael Ignatieff and NDP Leader Jack Layton. More than 5,000 people attended Thursday's remembrance mass.

The church's cathedral, which holds about 2,000, was filled to capacity and mourners spilled over into the basement where they were connected to the proceedings via video link. Still more listened to the ceremony from outside the church, where the proceedings were broadcast over loudspeakers. Many were moved to tears.

"It is ever, very, very important for Canada to do this," said Jerzy Rozyski of Mississauga, who was carrying a large Polish flag. "I'm sorry. I am unable to speak more. It's a very sad situation."

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.

With files from CBC's Steven D'Souza