The blast occurred in Trandnestr, which has been de facto independent from the rest of Moldova since the collapse of the Soviet Union.
The bomb was detonated in a Mercedes minibus used as taxi which was waiting at a red light in Transdnestr's main city, Tiraspol, beside a trolleybus packed with commuters.
The state attorney for the Transdnestr republic, which is notinternationally unrecognised, said eight died and 26 were injured, 20 of them required hospital treatment.
Tansdnestr television showed that all that remained of the red minibus after the explosion was a twisted heap of metal. The roof was ripped off and thrown almost 100 metres.
Oleg Belyakov, self-styled deputy interior minister of Transdnestr, pointed a finger at the Western-backed authorities in Moldova, which lost control of pro-Russian Transdnestr in a 1992 war but continues to claim sovereignty.
"We do not exclude the possibility of terrorism," Mr Belyakov said. "Given the worsening of relations with Moldova, the most likely explanation is that the explosion was organised by (Moldova's) special services with the aim of spreading panic among the people of Transdnestr."
There was no immediate reaction from officials in the Moldovan capital, Chisinau. Vladimir Voronin, the president of Moldova, offered his condolences and promised to cooperate with local authorities in Transdnestr.
The United States has condemned those behind the bloody bus bombing.
"Our thoughts are with the victims and their families. We stand ready to assist authorities investigating this terrible incident," US State Department spokesman Sean McCormack said in a statement.
Transdnestr is a Russian-majority region of eastern Moldova, which broke away in 1990 when the latter declared Romanian as its official language.
Transdnestr, stretching 460 kilometres along the border of southwest Ukraine, has only been recognised by Russia.