At least 26 people have died in a bus crash in the Mexican state of Guerrero. More than 20 people were injured, BBC reports.

The bus slid off the road into a ravine in wet conditions while on its way to a rally in the town of Buenavista de Cuellar, state authorities said.

Most of the passengers were wearing T-shirts bearing the logo of the left-wing Labour Party (PT), a Red Cross official told Reuters.

Several children were also reported to be among the dead.

Police, members of the military and emergency services were dispatched to the scene.

The passengers had been on their way to the closing rally of the PT's candidate in municipal elections.

Left-wing presidential candidate Andres Manuel Lopez Obrador and front-runner Enrique Pena Nieto both expressed their condolences to the families of the victims.

Mexico will hold presidential elections on 1 July.