Authorities confirmed that four people had so far been confirmed as dead and dozens had suffered injuries when the train came off the tracks at a bridge 12 km into the journey.
The Portuguese driver of the train and the ticket inspector, a man from Vigo, were reported to be among the confirmed fatalities.
Health authorities confirmed that around 47 people were being treated for injuries sustained in the crash. Of those hurt, 42 were described as suffering "light injuries" while seven were in a "serious condition".
The train was operated by Portuguese state-run company Comboios which shares services with Spanish rail operator Renfe and was carrying 69 passengers.
Renfe confirmed on Twitter that there had been a derailment, expressed condolences to the victims and their relatives and promised more information soon.
INFO. Descarrilo en Porriño del tren Vigo-Porto. Renfe expresa su pesar por las víctimas y sus familias. Informaremos.— Renfe (@Renfe) September 9, 2016
Renfe confirma la muerte de dos personas, uno el maquinista, en el descarrilamiento de un tren en Pontevedra. pic.twitter.com/hd4ngGXWPD— Marco Aracama (@MarcoAracama) September 9, 2016
The mayor of O Porriño, Eva García de la Torre, told reporters that the crash occured in an area where construction was underway to alter the tracks to allow for a high speed rail link.
The derailment occured on approach to the station in O Porriño where the speed is limited to 30 km/h. Passengers have emphasized that the train was not going at speed when the derailment occured, according to a report in the local newspaper La Voz de Galicia.
The first of three carriages came off the tracks and could be seen lurching to one side, the front of the train completely mangled. Photographs appeared to show that it had crashed into an electricity pylon.
Ramon Gonzalez, a passenger on the train who was interviewed by Spanish television, said there was a "very strong bang."
"It was full of black smoke," he said over the phone. "It's a straight line... so this isn't normal.
"There are still some injured here but very not serious, the seriously injured were quickly evacuated."
Adif, the company responsible for maintaining tracks across Spain's rail network said it had opened an investigation to determine the cause of the crash.
#DescarrilamientoOPorriño Adif ha abierto una investigación para determinar las causas del accidente ferroviario— Adif (@Adif_es) September 9, 2016
Spain's acting prime minister, Mariano Rajoy tweeted a message of condolence.
Dolor y tristeza por el accidente de O Porriño. Mi pésame a las familias de los fallecidos y deseos de pronta recuperación a los heridos. MR— Mariano Rajoy Brey (@marianorajoy) September 9, 2016
Three years ago, Galicia was the site of one of Spain's deadliest rail accidents when on July 24th 2013 a passenger train derailed as it approached the city of Santiago de Compostela killing 79 people and injuring 140.
The train was travelling over twice the 80 km/h speed limit and the driver was on the phone and consulting a map when the carriages careened off the tracks on a curve.