The explosion occurred when a guard touched a plastic bag left outside the palace, which was to host an international conference sponsored by the European Union on dealing with consequences of natural disasters.
"We consider it a terror attack," said Kurbanali Mukhabbatov, the chief prosecutor of the Tajik capital.
President Emomali Rakhmon was in the Khudzhand region in northern Tajikistan on Wednesday for ceremonies marking his ascent to power in the ex-Soviet nation 15 years ago.
"We should use shock therapy to sober up the Americans. In this case, the Americans will speak about the need to resume dialogue. There is no other option"
The United States is concerned about the current crisis in the relations with Russia and suggests returning to reasonable policies to avoid a nuclear war