The strike occurred at around 10 a.m.
Ukraine's National Police said a Russian projectile hit a street where two scheduled buses were traveling at the time. An apartment building was also damaged.
The Donetsk Regional Prosecutor's Office said Russian forces likely used a UMPB D-30SN guided aerial bomb. The exact type of weapon is still being established.
President Volodymyr Zelenskyy said a Russian aerial bomb struck a public transport stop in the city.
The death toll initially stood at 12 before rising rapidly as emergency crews worked at the scene.
By 12:47 p.m., prosecutors and Donetsk Oblast Governor Vadym Filashkin reported 30 people killed and 18 injured.
Fires broke out in two buses and on an open area following the strike. Emergency crews extinguished the fires and provided assistance to the injured.
Suspilne published footage from the scene showing the aftermath of the attack.
"Thirty-three lives that Russia took in a single moment," Filashkin said after the updated casualty figures were released.
Earlier, when 12 deaths had been confirmed, he wrote: "These are not 'consequences of war' or dry statistics. These are people killed by the Russian army."
The Kramatorsk City Council said Russian forces had struck civilian passenger transport carrying people on a regular route.
"Russia once again struck a place where there was clearly no military target. A scheduled bus. Civilian transport. People who were simply going about their business," the council said.
Public transport halted in Kramatorsk and Sloviansk
Authorities ordered a complete suspension of public transport services in Kramatorsk and neighboring Sloviansk following the attack.
Filashkin said the decision was prompted by what he described as deliberate Russian attacks on civilian transport.
"Under these circumstances, we have no right to risk the lives of passengers and drivers," he said.
The governor again urged residents who remain in Donetsk Oblast to evacuate to safer parts of Ukraine.
Ukraine's Air Force had warned at around 9:46 a.m. of Russian guided aerial bombs heading toward Donetsk Oblast.
The Donetsk Regional Prosecutor's Office opened a war crimes investigation into the attack.
The Oct. 8 strike came amid intensifying Russian drone and guided-bomb attacks on Kramatorsk and other cities in Ukraine's defensive belt in Donetsk Oblast.
On Oct. 6, a Russian FPV drone struck a patrol police vehicle in Kramatorsk. Three officers were injured, while 40-year-old patrol officer Artem Kovtun died from his injuries.