SYDNEY/MELBOURNE (Reuters) - Police have arrested a 20-year-old man suspected of planning to use an automatic rifle for a mass shooting on New Year s Eve in downtown Melbourne, Australia s second largest city. Victoria state police said on Tuesday that the man was acting alone and had not been able to acquire a firearm before his arrest late on Monday.  Victoria Police Deputy Commissioner Shane Patton said the man, an Australian citizen with Somalian parents, had been monitored by authorities since the beginning of the year.   He s accessed documents produced by al-Qaeda Arabian Peninsula which is a guide book in respect to how to commit a terrorist act and also how to use firearms, guns and handguns and rifles,  Patton said at a press conference in Melbourne.  We are alleging that ... he is a sympathiser of ISIS.     The man had planned to shoot as many people as possible at Melbourne s Federation Square, which swells with crowds on New Year s Eve as the focus of the city s celebrations, Patton said. He was charged with preparing to commit a terrorist attack and gathering documents likely to facilitate a terrorist act, according to the Australian Federal Police Australia, a staunch U.S. ally that sent troops to Afghanistan and Iraq, has been on heightened alert since 2014 for attacks by home-grown militants returning from fighting in the Middle East or their supporters. A gunman in a deadly 2014 Sydney cafe siege boasted about links with Islamic State militants, although no direct ties with the group were established. The shooting murder of a police accountant by a 15-year old boy in 2016 was claimed by Islamic State. Around a dozen significant plots have been foiled since the alert was issued, according to officials, including a plot to attack prominent sites in Melbourne last Christmas Eve and a plan to blow up an Etihad Airways flight from Sydney to Abu Dhabi using a bomb disguised as a meat mincer.          