"It is very important to show that the big majority of the Swedish population is against the right-wing extremists like the Sweden Democrats," Per Branevig, 33, told AFP, adding that he had voted for the Social Democrats in Sunday's election.
"It has been a big shock for me that they got so many votes," he said of the anti-immigrant party that secured 5.7 percent of the vote.
At around 6pm up to 10,000 people, some waving banners stating "Yes to togetherness, No to racism" and "No racists in parliament," gathered in Stockholm's Sergels Torg, according to a police estimate.
The peaceful gathering was spontaneously organised by a 17-year-old girl from the Stockholm suburb of Sollentuna, Felicia Margineanu, who was so disappointed by the election results she posted a protest call on her Facebook page.
Sweden's ruling centre-right coalition won the most votes Sunday but fell short of a overall majority, as the Sweden Democrats nearly doubled their result from the last election and entered parliament for the first time, landing it in a key position in the Riksdag.
"We don't like those racists in the Riksdag. Being here shows the government that we care, that we don't want them (the Sweden Democrats) in parliament," said 18-year-old Younes Sedik, wearing a black T-shirt with orange lettering stating "I'm a Muslim. Don't panic!".
Standing near banners pointing out that "9,043,222 Swedes did not vote for the Sweden Democrats yesterday," and that "94.3 percent of the people are not racists," 21-year-old Thomas Zebuehr said he was "really unhappy" the extremists passed the four-percent barrier for entering parliament.
"I'm not sure what should be done, but something has to be done," he told AFP.
A spontaneous demonstration against the Sweden Democrats also gathered in Sweden's second largest city Gothenburg Monday evening, with up to 1,000 people participating, the TT news agency reported.
Another large demonstration is planned to be held in Stockholm on October 4th.