The demonstration was organised by groups close to the Kurdistan Workers' Party (PKK), outlawed by Turkey, which this week warned against "terrorists" being allowed to demonstrate in Sweden.

Turkey's President Recep Tayyip Erdogan has so far blocked Sweden's NATO membership, accusing Stockholm of being a haven for the Kurdish activists.

To address his concerns, Sweden passed a new law that criminalises "participation in a terrorist organisation".

"They are after the Kurds in Sweden," Tomas Pettersson, spokesperson for the Alliance Against NATO, told AFP at the protest, titled "No to NATO, No Erdogan Laws in Sweden."

Petterson added that the idea behind the law is "to have an arrest and a trial and a victim," so that Erdogan "will then let Sweden into NATO".

Protesters waved numerous PKK flags, along with signs reading "No to NATO."

"Our membership in NATO would cause a lot of blackmail from Erdogan," former Swedish MP Amineh Kakabaveh told AFP.

A spokesman for Erdogan on Tuesday said it was "completely unacceptable that PKK terrorists continue to operate freely in Sweden" and urged Swedish authorities to block the protest.

Even though the PKK is also considered a terrorist organisation in Sweden -- as in the rest of the EU -- its supporters are generally allowed to protest in public.

Sweden and Finland dropped decades of military non-alignment and applied to join NATO in response to Russia's invasion of Ukraine.

Finland formally joined in April, however Turkey and Hungary have yet to ratify Sweden's membership bid.

Sweden's justice minister reiterated on Friday that the new law is not aimed at attacking freedom of speech.

Foreign Minister Tobias Billstrom on Thursday hailed the new legislation as Sweden's last step under an accord signed with Turkey last year for Ankara to ratify Stockholm's membership.

After meeting Erdogan in Turkey, NATO chief Jens Stoltenberg on Sunday called on Ankara to drop its opposition to Sweden's bid, saying Stockholm has addressed security concerns.

Ankara suspended negotiations with Sweden in outrage after protests in January that included a Koran burning outside Turkey's embassy in Stockholm.