Far-right leader Giorgia Meloni, leading the race to become Italy's next prime minister, was accused on Monday of shameful electioneering by her rivals after posting a video of a Ukrainian woman being raped by a migrant in an Italian city.

The 55-year-old woman was assaulted on a pavement in the city of Piacenza early Sunday by an asylum seeker from Guinea, local officials said. The incident was videoed by someone in a flat overlooking the street and the assailant was arrested.

Police confirmed the arrest and said the man was being detained as the investigation continued.

Meloni, whose Brothers of Italy party heads the polls ahead of a Sept. 25 national election, tweeted the video, which had been posted on a newspaper website with the image blurred but the woman's cries clearly audible.

"One cannot remain silent in the face of this atrocious episode of sexual violence against a Ukrainian woman carried out in daytime in Piacenza by an asylum seeker," Meloni wrote.

"A hug to this woman. I will do everything I can to restore security to our cities."

The tweet drew a barrage of criticism online, as well as from Meloni's political opponents.

"It is indecent to use images of a rape. Even more indecent to do so for electoral purposes," Enrico Letta, head of the centre-left Democratic Party (PD), wrote on Twitter.

Another centrist leader, Carlo Calenda, called Meloni's Tweet "immoral". Igiaba Scego, a prominent Italian writer of Somali heritage, accused Meloni of exploiting the rape victim.

"Offered up as clickbait voyeurism instead of being protected. This electoral campaign is horrendous," she wrote.

Meloni, who has called for a naval blockade of north Africa to prevent migrant boats from setting sail, said on Facebook her rivals had used the rape to attack her while ignoring the victim to avoid addressing what she called the immigration emergency.

Letta's post had said: "Respect for people and victims always comes first" and Calenda wrote "only the victims matter".