Police said that the militants launched their attack as the neo-Nazi procession in the small town of Kungälv neared the Nytorget Square where they were set later to hold their rally.
“As the NMR was passing the place where the counter-demonstrators were located, bangers were thrown at the procession. Cobble stones were also thrown at police on the scene,” the local police wrote on their website.
The protesters threw bangers at the Nordic Resistance Movement as they passed. Photo: Björn Larsson Rosvall/TT
Police spokesperson Anna Göransson told the TT newswire that that 18 counter-demonstrators had been arrested as police brought the situation under control, protecting the marchers with police cars and police horses.
Her colleague Christer Fuxborg told the Expressen newspaper that those who had thrown stones at police horses had been “unbelievably cowardly”.
After the violence had been largely brought under control, counter-demonstrators continued to attempt to disrupt the neo-Nazi rally in Nytorget square by singing loudly and making other noises, the newspaper reported.
NMR's leader Simon Lindberg spoke over the singing of the counter-demonstrators. Photo: Björn Larsson Rosvall/TT
According to the police, about 500 counter-demonstrators launched a procession at about midday, despite not having received permission from the police in advance.
Some were dressed as clowns, some carried rainbow flags but others were dressed in the black clothing and face masks of the militant Antifa group, leading police to issue a notice forbidding anyone from wearing a mask.
According to the police, about 300 neo-Nazis took part in the official procession. Two people were arrested for knife crimes before the procession even started.
Sweden's culture minister Amanda Lind, who represents the pro-immigration Green Party, was in Kungälv to join one of the official counter-demonstrations.
“It is so important that we show how many there are of us and that there are more of us than of the Nazis,” she told the TT newswire. “I do not believe that we will beat the Nazis by staying quiet.”
Annie Lööf, the leader of the Centre Party, was also set to make her first ever May 1 Labour Day speech in the town, which she told Expressen she would use to highlight her party's opposition to illiberal far-right groups.
Some protestors wore the black clothes and carried the flag of the militant left-wing group Antifa. Photo: Björn Larsson Rosvall/TT
Police said between 130 and 150 supporters of the neo-Nazi group had joined a second demonstration in the town of Ludvika.
Nordic Resistance Movement supporters starting their march in Ludvika. Photo: Ulf Palm/TT
Kungälv, north of Gothenburg, and Ludvika in Dalarna in central Sweden, are regarded as the neo-Nazi group's strongholds, although in September's election, the group fell far short of attracting sufficient votes to get members elected to the local council in either town.
Nordic Resistance members gathered in a car-park in Kungälv. Photo: Björn Larsson Rosvall/TT
Police spokesperson Christer Fuxborg said that police planned to closely watch the marchers to check that none of their banners or slogans broke hate crime laws.
“We have hate crime laws, so we are of course going to scrutinise carefully if what anything they do tips over into some kind of crime,” he said.
“We have many colleagues in place who are going to try and make sure that it doesn't go over the border.”
The neo-Nazi group's marches have several times in the past descended into violence.
When the neo-Nazi group marched in Gothenburg in September 2017, several activists broke away from the route designated by police so that they could protest against the Gothenburg Book Fair, which had banned some far-Right publishers. They then attacked the police, leading several to be later charged for rioting.
In the summer of 2017, a scuffle broke out at the Almedalen political festival, with NMR members shouting “treasonist” during speeches of Anna Kinberg Batra, the leader of the Moderate Party, and of Isabella Lövin and Gustav Fridolin, the two leaders of the Green Party.