Newly elected MEP Magdalena Adamowicz is on a mission to craft new Europe-wide laws on hate speech, and she has more moral authority to make the demands than most.

Her husband, Paweł Adamowicz, for more than two decades the mayor of the northern Polish city of Gdańsk, was stabbed while on stage at a charity event in January and died soon after. He had been a vocal critic of Poland’s nationalist government and an advocate of tolerance in the city, which had long been a stronghold of the liberal opposition.

“It’s my mission and my obligation to do this. I don’t want the death of my husband to be in vain,” she said during a recent interview in Gdańsk, to which she wore a black T-shirt emblazoned with the slogan “Imagine there’s no hate”, which has become her trademark look.

In the week after her husband’s murder, Adamowicz was in shock and did not participate in public events or rallies, she said. Afterwards, though, she was struck with a sense of purpose, and decided to stand in European parliamentary elections in May. She easily won a seat, and said working on new Europe-wide hate speech legislation is the main task for her mandate.

“The problem is not only in Poland, the problem is worldwide. We need new regulations to define hate speech. In Poland we have some regulations but they are limited and they are clearly not working. Hate is everywhere,” she said.

Polish society is split between supporters of the ruling Law and Justice (PiS) party, and the more liberal opposition. As an advocate for the rights of LGBT people, immigrants and other minorities, Paweł Adamowicz was often in the crosshairs of the pro-PiS media.

While there is no proof of a direct link between increasingly fraught political rhetoric in Polish politics and the attack, and there is reason to believe the perpetrator suffered from mental illness, many people in the aftermath claimed the climate of hate played a part, and the politician’s widow agrees.

“I believe my husband was the victim of hate speech. I don’t want to blame anyone personally. I do not feel any hate to the man who did it … a person who is under the influence of hate speech and takes it seriously and then commits a crime because of the hate, they are kind of victims too,” she said.

For some time after the murder, a kind of truce held. “The murder of Adamowicz is a huge tragedy for all of us. It’s a great evil, which evokes condemnation, sadness and sorrow,” tweeted the prime minister, Mateusz Morawiecki, who is a PiS politician.

Radosław Fogiel, an adviser to the PiS chairman, Jarosław Kaczyński, blamed the opposition for beginning the split in Polish society but admitted that the atmosphere was unhealthy. “Our electorate and the other side’s electorate sometimes tend to hate each other more than the politicians do,” he said.

Now, with crunch parliamentary elections due in October that will determine whether PiS keeps its hold on power for another term, government-controlled media outlets are again ratcheting up the rhetoric.

Aleksandra Dulkiewicz, the deputy mayor of Gdańsk under Adamowicz who won a March election to replace him, said she receives threatening letters, phone calls and comments on social media accounts on a daily basis. She now has two bodyguards.

The day before she met the Guardian, during a break from a sitting of the city parliament, she received one message saying “You will hang” and another with her mocked up in the uniform of a Nazi soldier. Every threat is reported to police, and according to Polish media, as of early August police were investigating eight separate threats against her.

“This atmosphere is not normal. This is not the way a democratic society should be organised,” she said. She is backing Adamowicz’s campaign for new laws.

Adamowicz said part of her plan was to make companies such as Facebook held more responsible for hate speech published on their platforms, not only the users.

“If you are a journalist and you write some fake information in your newspaper, you and your editor are both responsible, so I think we have to do the same online,” she said. She has already met Facebook’s chief operating officer, Sheryl Sandberg, in Palo Alto to discuss what the social media giant is doing to combat hate speech, and has meetings planned with Twitter in Brussels later this month.

Adamowicz realises that the proposals are not without controversy, given the difficult and knotty issue of where freedom of speech stops and hate speech begins, and she expects any legislation to take several years to discuss and push through. But she believes that in the end, it will be possible to change the culture.

She compared it to improvements in violence and discrimination among British football fans over recent years. “After changing the law, some changes in stadium infrastructure, implementing bans and increasing education, you can see that there is a very big change,” she said.

Additional reporting by Monika Prończuk