One reporter said on Monday he had seen the head of man lying on the road and car parts scattered all around.
Four people in a taxi were also caught up in the blast and suffered concussions about one kilometre from the palace of President Imomali Rakhmonov.
Speaking from the scene, Emergencies Minister Mirzo Ziyoyev said there were no other reports of casualties.
But several cars parked nearby had been burned by the blast, which shattered windows in surrounding buildings and mangled metal grates in front of the windows of the emergencies ministry building.
The area was cordoned off by police and firefighters were at the scene.
Tajikistan, an impoverished Muslim state of 7 million which neighbours Afghanistan, is due to hold a parliamentary election on 27 February.
The country, which lost more than 100,000 people in a 1992-97 civil war, has since been relatively stable although kidnappings continue and crime is rampant.