The leader of South Korea's ruling party was hospitalized on Monday after being attacked while campaigning for this week's presidential election, a race already overshadowed by controversy and in which early voting has been marred by irregularities.

Democratic Party leader Song Young-gil was attacked in Seoul by a man wearing a traditional robe who approached him from behind and struck him on the head with a small hammer, a video uploaded to YouTube by a party campaigner showed.

Reuters could not independently verify the images, but party officials said Song was in a stable condition and that a suspect was in police custody.