"They are after the Kurds in Sweden," Tomas Pettersson, spokesperson for the Alliance Against NATO, told AFP at the protest.

He added that the idea behind the law is "to have an arrest and a trial and a victim" so that Turkish President Recep Tayyip Erdogan "will then let Sweden into NATO".

The new law that came into effect on June 1 criminalises "participation in a terrorist organisation" as part of Sweden's effort to beef up anti-terror legislation, a key demand from Turkey to approve Stockholm's stalled NATO membership bid.

Ankara voiced its discontent earlier this week regarding the scheduled protest -- titled "No to NATO, No Erdogan Laws in Sweden" -- organised by groups close to the Kurdistan Workers' Party (PKK), which is outlawed by Turkey.

On Sunday, the protesters were observed waving numerous PKK flags, along with signs stating "No to NATO."

A spokesman for the Turkish presidency on Tuesday said it was "completely unacceptable that PKK terrorists continue to operate freely in Sweden" and urged Swedish authorities to block them from demonstrating on Sunday.

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'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.

"The fact is that the law is very vaguely written, it's extremely vague," Vilgot Karlsson from International Socialist Alternative told AFP at the protest.

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.

Erdogan, re-elected for five more years on May 28, has so far blocked Sweden's NATO membership, accusing Stockholm of being a haven for Kurdish activists Turkey considers "terrorists".

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