Ambulances and fire engines were rushed to the scene, the state-run Anadolu news agency reported. Television footage showed plumes of white smoke rising above.
The initial wounded toll was cited by the private NTV broadcaster, citing sources from the local governor’s office.
It was not immediately clear who carried out the attack. CNN-Turk channel said the bomb was detonated by remote-control.
Images of the vehicle published by the Dogan news agency showed a white bus with its windows blown out by the force of the blast, a fire burning still in the back of the vehicle and glass scattered on the roadside.
Turkey was hit in 2016 by a succession of attacks that left hundreds dead in the bloodiest year of terror strikes in its history.
The attacks were blamed on IS jihadists who had taken swathes of territory in neighboring Syria and Iraq as well as the outlawed Kurdistan Workers Party (PKK) who have battled the Turkish state in an insurgency lasting more than three decades.
An attack by a jihadist gunman on an elite nightclub in Istanbul just 75 minutes into New Year’s Day in 2017 left 39 people dead, mainly foreigners.
There has since been a lull in similar attacks, but tensions and security remain high in big cities. The PKK still carry out sporadic attacks against the armed forces in the southeast.
Mersin, a major port of around one million people, lies well to the west of the PKK’s main area of operation in the southeast.
The army has been combating the PKK in the Kurdish-majority southeast, in a relentless campaign to root out the group blacklisted by Turkey as well as its Western partners.