Vladimir Putin has signed the corresponding decree, the Kremlin press service said.
"As a result of the air crashes which happened on the night of Tuesday, August 24, 2004, in the Tula and Rostov regions, numerous victims are reported. Expressing sorrow at the deceased and presenting condolence to their relatives and friends, I declare August 26, 2004 as the Day of Mourning in the Russian Federation", the decree reads.
It notes that on the Day of Mourning state flags of the Russian Federation will be flown at half-mast across the country.
Cultural institutions and television and radio companies have been ordered to cancel entertainment events and transmissions.
The president has expressed condolences to the relatives and friends of those who died in the Tu-154 and Tu-134 air crashes.
"We are gathered here in connection with the tragedy of Tuesday, the crash of two planes. Before we get down to work I'd like to express condolences to the relatives and friends of the deceased", the Russian president told the conference with law-enforcers in the Kremlin on Wednesday night.
"I have signed the instruction appointing the transport minister as the chairman of the fact-finding state commission. I hope that the first steps in your work will be the unconditional provision of objective, trustworthy information about the accidents and help to all those in need of it", the president said addressing Transport Minister Igor Levitin.
The Russian head of state has assured that all the relatives of the deceased will get help.
"Everything depending on us must be done to support the people", Putin said.
He asked Prosecutor General Ustinov about the progress of the investigation. Ustinov said that a criminal case has been initiated and two teams led by hisdeputies have been set up.
Ustinov said: "There is information that the black boxes from the crashed planes have begun to be decoded".
"The law-enforcement agencies are considering several versions, such as an act of terrorism, technical and the human factor. None of them is being rejected", he added.
At that, the presidential conference continued without pressmen.
On Tuesday two jetliners, Tu-134 and Tu-154, crashed almost simultaneously in the Tula and Rostov regions. In preliminary information, 81 people have died.
