April 12 will be a day of national mourning for the victims of the plane crash in western Russia that killed the Polish president and 96 others, Russian President Dmitry Medvedev said in a television address to the Polish people on Saturday.

The president expressed Russia's condolences on the death of Polish President Lech Kaczynski, his wife and members of the Polish delegation in the air crash in the Smolensk region.

Medvedev stressed that he had ordered a thorough investigation of the causes of the crash.

"This work will be done in close interaction with corresponding Polish structures and agencies," he said. "All instructions have been given to this end."