At approximately 6.30am in the Abbot Drive area of Newtownards a bus was boarded by two masked, armed men. They poured fuel over the vehicle before setting it alight.
The bus driver escaped without injury but is said to have been left badly shaken.
The attack is believed to have been carried out by loyalists in opposition to the Northern Ireland protocol, which has created a trade barrier between Northern Ireland and the mainland UK.
The incident was condemned by DUP leader Jeffrey Donaldson.
"There was never any justification for people with guns on our streets and damaging property - there never will be," he wrote on Twitter. "Thuggery and terrorism will do nothing to remove the NI Protocol. Political action has secured progress and must be allowed to continue. Violence has no place in this."
"There is absolutely no place in our society for this criminality and violence," said Sinn Féin MP John Finucane, who described the attack as "dangerous, reckless and despicable."
"Our bus drivers are frontline public workers, our buses provide a critical community service," said SDLP Infrastructure Minister Nichola Mallon. "These cowards with covered faces have done nothing more than attack their own community."
Investigations are ongoing.
