Authorities in Turkey arrested 20 people on Saturday for allegedly having links to the Islamic State group, as part of the country’s tightened security arrangements ahead of the first anniversary of the New Year’s Eve attack at an Istanbul nightclub.

Police believe that those arrested were planning attacks in the city this New Year’s Eve as well, Anadolu news agency reported.

Fifteen of the 20 people arrested are foreigners, but police have not yet given information about their nationalities. They were arrested after the police’s anti-terrorism branch raided four locations in Istanbul.

The Turkish police had detained around 195 people on Thursday and Friday, according to Reuters. Public celebrations on New Year’s Eve have been cancelled or banned in some districts for security reasons, and almost 37,000 police officers will be on duty on the night – double the normal number.

On New Year’s Eve in 2016, an attacker dressed as Santa Claus, fired indiscriminately at patrons at Istanbul’s famous Reina nightclub, killing 39 people, including two Indians. The Islamic State group claimed responsibility for the shooting. Most of the victims were foreigners.