Violence in the run-up to Mexico's mid-term elections on 7 June has killed at least seven candidates and forced another 20 out of the race, battering the government's record on law and order.

Drug gangs battling for control of trafficking routes on the Pacific coast have murdered or intimidated candidates, while militant teachers opposed to education reforms have threatened to sabotage voting stations in much of south western Mexico.

The violence flies in the face of President Enrique Pena Nieto's pledge when he took office in December 2012 that his government would restore order to the country.

On 4 June in Oaxaca, disgruntled protesters ransacked an office of the ruling Institutional Revolutionary Party (PRI) and set documents and portraits of Pena Nieto ablaze.

Guerrero, where 43 trainee teachers were abducted and almost certainly massacred last year by a drug cartel in league with local police, has been hardest hit by the electoral violence, in spite of pledges by Pena Nieto to bring peace there.

On June 3, during a demonstration calling for justice for the missing 43, clashes broke out and a protester was injured.

"Aside from the regretful and condemnable acts in Guerrero and in Iguala last year, delicate expressions of social discontent have, for the first time, threatened to disrupt or stop the elections," said the president Of Mexico's National Electoral Institute, Lorenzo Cordova.

"We have not requested the assistance of the police because we considered that it is not convenient to militarise the elections, these protests have complicated our field work," he added.

Along with the seven candidates, at least nine campaign officials have been killed in different areas of the country.

Still, in 7 June's legislative elections, polls forecast the ruling PRI will retain a slim working majority in the lower house of Congress, partly as the main opposition parties are riven by divisions.

Pena Nieto is a few seats short of a majority in the Senate, which is not up for re-election until 2018. Under his presidency, the murder rate has fallen in troubled parts of northern Mexico, but violence has jumped in western areas, including the country's second biggest city, Guadalajara.