At least 20 people, alleged to have attacked police during May 1 protests, have been taken into custody Wednesday in a broad anti-terror operation in Istanbul, security officials said.Sources within the anti-terror branch directorate of the Istanbul police department said they had launched a helicopter-supported operation against suspected members of the Maoist Communist Party (MPK).Members of the group are believed to have attacked police with stones, sticks and Molotov cocktails nearly a month ago.About 800 policemen, including riot police forces and special operations teams, raided 30 addresses in Istanbul's 10 districts.Police sources said that they had confiscated two long-barreled rifles, one pistol and a large number of organizational documents.A total of 203 illegal protesters were taken into custody in Istanbul during May Day demonstrations held on May 1, according to the province governor Vasip Şahin.