Senator Monika Piątkowska, who is backed by the ruling Civic Coalition (KO) and Polish People's Party (PSL), said on Saturday that numerous social media accounts were being used to discredit her, her team and her allies.
She described the content as "full of hate" and called it a form of violence.
She said the campaign was targeted at specific groups of residents and funded by unknown sources.
"It's not only an attempt to brutalise public debate, but perhaps also a way of circumventing the law, and certainly exploiting loopholes," she told reporters.
Piątkowska said three of the accounts running targeted adverts had generated more than 3 million views.
She added she would take "concrete steps" in response and urged rival candidates to run a clean campaign.
Piątkowska was joined at the briefing by Justice Minister Waldemar Żurek, who studied, lived and worked in Kraków for many years.
He said inflammatory rhetoric was spreading during the campaign and could lead to hate crimes.
"I appeal to all candidates: let us obey the law and play fair," he said, warning that violating the rules could have serious consequences for individuals, institutions and political parties.
Kraków, a city of around 1 million people, holds its mayoral election on September 27, after residents voted to remove mayor Aleksander Miszalski of the KO in a referendum in May.
Eight candidates have been registered.
(ał)
Source: PAP