There were conflicting reports on Wednesday as to whether the bomb was hidden in a car or a motorcycle, and photographs showed the damaged remains of both at the scene.
The police offered no immediate motive for the attack, but Mr. Mosiychuk was a pugnacious figure engaged in the politics of the war in eastern Ukraine who had publicly baited and insulted Russian politicians. He had posted a video on YouTube threatening to kill the leader of the Russian region of Chechnya, for example.
Assassinations in Ukraine have been carried out with bombs and pistols, and some of the attackers have demonstrated careful preparation.
In one instance, a would-be assassin got close to his targets, a husband and wife who were soldiers in a Ukrainian paramilitary unit, by posing as a journalist seeking an interview for the French newspaper Le Monde.
In that attack, the man carried a gift box to the interview, saying it was a present from the newspaper’s editors. In fact, he was carrying a gun in the box. But the plot was foiled when the woman pulled her own gun and shot the man, according to the accounts of the two paramilitary soldiers and the Ukrainian authorities.
In another case last March, an assassin shot and killed a former member of the Russian Parliament who had defected to Ukraine. The assailant was then killed by the ex- lawmaker’s bodyguard.
The explosion on Wednesday was the fifth bombing in Ukraine in the past year and a half that involved some sort of vehicle. In September, a car bomb in Kiev killed an ethnic Chechen fighting on the Ukrainian side. The blast severely wounded a woman who was also in the car, but a 10-year-old child in the back seat was largely unscathed. That attack, the police said, might have been linked to a criminal dispute.
Another car bomb killed the journalist Pavel Sheremet in July 2016, and two others killed Ukrainian security service officials this year, in June and March.