Poland has declared a week of national mourning following the death of its president and many of its political and military leaders in a plane crash in Russia.

In one of the country's worst days since the second world war, 97 people were killed when their plane went down in heavy fog.

The group had been travelling to western Russia for a memorial service marking the 70th anniversary of the Katyn massacre, in which Russian forces killed more than 20,000 Polish prisoners of war.

Church leaders and some of the families of those who had died in the massacre were also on board.

Vladimir Putin, the Russian prime minister has promised a swift investigation into the incident and it leading the probe himself.

Al Jazeera's Jonah Hull reports from Warsaw.