Hong Kong's security chief on Monday welcomed China's plans to impose a national security law on the financial hub, saying it was needed to defeat "terrorism" and calls for independence. Police fired tear gas and water cannon at thousands of Hong Kong pro-democracy protesters who gathered on Sunday against the controversial law, in the most intense clashes for months. "Terrorism is growing in the city and activities which harm national security, such as 'Hong Kong independence', become more rampant," security minister John Lee said in a statement welcoming the planned law. The statement was part of a flurry of coordinated messages from the city's different security services embracing the law, which will be written in Beijing and not go through semi-autonomous Hong Kong's legislature. Police chief Chris Tang cited 14 recent cases where explosives had been seized and said the new law would "help combat the force of 'Hong Kong independence' and restore social order". As the demonstrators and police were facing off in the semi-autonomous financial hub, Chinese Foreign Minister Wang Yi insisted in Beijing that the proposed law must be imposed "without the slightest delay". The planned legislation – expected to ban treason, subversion and sedition – comes after Hong Kong was shaken last year by months of massive, often-violent protests, and repeated warnings from Beijing that it would not tolerate dissent. Campaigners view the security law proposal as the most brazen move yet by Beijing to end free speech. "People may be criminalised only for words they say or publish opposing the government," 25-year-old protester Vincent told <em>AFP</em>. "I think Hong Kongers are very frustrated because we didn't expect this to come so fast and so rough. But... we won't be as naive as to believe that Beijing will simply sit back and do nothing. Things will only get worse here." Of particular concern is a provision allowing Chinese security agents to operate in Hong Kong, with fears it could spark a crackdown against those voicing dissent against the mainland's communist rulers. As the number of protesters swelled, police fired tear gas and pepper spray to try and disperse the crowd, and later deployed water cannon and armoured vehicles against pockets of protesters. The comments from Hong Kong's security chief came as a video of protesters beating a lawyer was seized on by China's state media. Video shot during Sunday's rally showed a man with blood on his face scrambling to escape some half a dozen protesters who were kicking and hitting him with umbrellas. At least 180 people were arrested, police said, the majority in Causeway Bay and Wan Chai districts. Other protesters were detained at a smaller demonstration in Tsim Sha Tsui. The Hong Kong government condemned the "extremely violent and illegal acts" of the protesters and said they reinforced "the need and urgency of the legislation on national security". It also accused protesters of injuring at least four police officers. The Hong Kong pro-democracy movement had fizzled at the beginning of 2020 as arrests mounted and, later, large gatherings were banned to stop the coronavirus. More than 8,300 people have been arrested since the protests erupted last year. Around 200 were detained during small rallies at malls on Mother's Day earlier this month. Hong Kong residents enjoy rights – including freedom of speech – unseen on the Chinese mainland, as well as its own legal system and trade status. Fears had been growing for years that Beijing was chipping away at those freedoms and tightening its control on the city, and campaigners have described the new proposal as the most brazen move yet. "I'm very scared, but I still have to come out," said protester Christy Chan, 23. "Aside from being peaceful, rational and non-violent, I don't see many ways to send out our messages." China's legislature is expected to rubber-stamp the draft resolution on Thursday, before the details are fleshed out at another meeting on a later date. China warned on Monday that it will take countermeasures if the United States insists on undermining its interests regarding Hong Kong, following the latest comments from Washington about possible sanctions over new national security legislation for the city. President Trump condemned the proposed law last week and threatened to respond “very strongly” if Beijing pursues it. Chinese foreign ministry spokesman Zhao Lijian told reporters during a briefing that the United States is trying to harm China's national security and said Beijing has lodged stern representations with Washington over White House National Security Adviser Robert O'Brien's comments that the security law for Hong Kong could lead to US sanctions.