Twenty people suspected of links to the ISIS terrorist group were detained in raids by Turkish police in the city of Istanbul. The raids by the anti-terrorist branch of the Istanbul police took place on Saturday and targeted four locations across the city, the police said in a statement on Sunday. The detainees include 15 foreigners, but their nationalities were not disclosed. The raids came ahead of the one-year anniversary of a nightclub attack in which dozens were killed. The state-run Anadolu news agency said the suspects were believed to have been planning an attack during New Year’s Eve celebrations in Istanbul. Turkish police have been ramping up operations against suspected ISIS militants in recent days, detaining around 195 on Thursday and Friday. During New Year’s celebrations a year ago, a man with an assault rifle shot dead 39 people including Turks and visitors from several Arab nations, India and Canada at an exclusive nightclub in Istanbul, the country’s biggest city. ISIS claimed responsibility for that shooting, one of a series of attacks believed to have been carried out by the extremists in Turkey in the last couple of years. A trial of those allegedly involved in the nightclub attack began this month. On New Year’s Eve, the number of police officers on duty in Istanbul will be doubled to 37,000 and public celebrations will be canceled or banned in some districts for security reasons, officials have said. Many roads in central Istanbul districts will be closed, while heavy vehicles will not be allowed to enter some areas, police have also said. On Friday, Turkish authorities rounded up 75 people in Istanbul and Ankara suspected of links to ISIS, Anadolu said. The police detained 46 people -- 43 of them foreigners -- in 10 different areas of the city. Without giving any details, it said they are suspected of planning attacks over the New Year period. Anadolu said that even before the latest arrests, a total of 120 ISIS suspects had been detained in operations nationwide. The authorities have banned any New Year celebrations in Taksim Square in the heart of the European side of the city while a similar measure has been imposed for the lively district of Besiktas. The district of Sisli -- home to Istanbuls most upmarket shopping and residential areas -- has also scrapped New Year celebrations on security grounds.