Leading Hong Kong democracy campaigners were disqualified Thursday from upcoming elections after four student activists were arrested for social media posts, sparking warnings of a new "terror" under authoritarian China. The moves were the latest blows against the semi-autonomous city's democracy movement, which has been under sustained attack from China's Communist Party rulers. China imposed a national security law last month on Hong Kong outlawing subversion, which it warned was a "sword" hanging over the head of democracy protesters. In some of the most significant developments since the law was imposed, 12 democracy activists were on Thursday disqualified from legislative elections due to be held in September. "Beijing shows a total disregard for the will of the Hongkongers, tramples upon the city's... autonomy," Joshua Wong, one of Hong Kong's highest-profile activists who was among those disqualified, wrote in a tweet. Wong described the move as "the biggest-ever crackdown" on the city's pro-democracy movement. The democracy campaigners had been hoping to win a first-ever majority in the partially elected legislature, which is deliberately weighted to return a pro-Beijing chamber. In a statement Hong Kong's government listed political views that required disqualification -- including criticising Beijing's new security law, campaigning to win a legislation-blocking majority and refusing to recognise China's sovereignty. The Liaison Office, which represents Beijing in Hong Kong, hailed the disqualifications, describing the candidates as "unscrupulous delinquents" who had "crossed the legal bottom line" with their political views. Chris Patten, Britain's last colonial governor in Hong Kong, accused Beijing of carrying out "an outrageous political purge". "It is obviously now illegal to believe in democracy... This is the sort of behaviour that you would expect in a police state," he added. - Arrested students - The disqualifications came after four students -- aged between 16 and 21 -- were arrested on Wednesday night for social media posts deemed to breach the new security law. The four were all former members of Student Localism, a pro-independence group that announced it was disbanding its Hong Kong branch the day before the security law was enacted. Police said they were arrested on suspicion of organising and inciting secession through comments made on social media posts after the law came in. Student and rights groups condemned the arrests, saying they heralded the kind of political suppression ubiquitous on the Chinese mainland. "Hong Kong has fallen into the era of white terror," the Student Unions of Higher Institutions, which represents 13 student unions, said in a statement overnight. Nathan Law, a democracy campaigner who went into exile after the law was imposed, condemned both the arrests and disqualifications. "White terror, politics of fear dispersed in Hong Kong," he said, referencing a Chinese idiom to describe political persecution. "The Chinese Communist Party's strategy is apparently aiming to suppress all forms of resistance in Hong Kong with huge fear and intimidation," he added. - 'Draconian' law - The security law gave China's rulers far more direct control over Hong Kong, which was supposedly guaranteed 50 years of freedoms as part of the 1997 handover agreement with Britain. But last year the city was rocked by seven straight months of huge and often violent pro-democracy protests. Beijing said the national security law was needed to end that unrest and restore stability. It targets four types of crime: subversion, secession, terrorism and colluding with foreign forces -- with up to life in prison. Critics, including many Western nations, say it has demolished the "One Country, Two Systems" model promised by Beijing in the handover agreement. The law bypassed Hong Kong's legislature and its details were kept secret until the moment it was enacted. It empowers China's security agents to operate openly in the city for the first time. Beijing has said it will have jurisdiction for particularly serious cases, toppling the legal firewall that has existed since the handover between Hong Kong's independent judiciary and the Chinese mainland's party-controlled courts. China has also claimed it can prosecute anyone anywhere in the world for national security crimes. On the mainland Beijing routinely uses similar national security laws to crush dissent. The first arrests in Hong Kong came a day after the law was enacted against people who possessed pro-independence flags and slogans critical of Beijing -- including a 15-year-old girl. At least 15 people have now been arrested under the new law since it was enacted on June 30.