Hong Kong's leader has promised an investigation after suspected triad gangsters on train passengers on Sunday, after a night of bloody attacks opened new fronts in the political crisis now deepening across the city. Some fear the escalating violence could prompt an intervention by China’s People’s Liberation Army. Screams rang out in the rural Yuen Long station on Sunday as men clad in white T shirts and armed with poles stormed a train, attacking passengers, according to footage taken by commuters and Democratic Party lawmaker Lam Cheuk-ting. Some passengers had been at an earlier anti-government march and the attack came after several thousand activists surrounded China's representative office in the city, later clashing with police. Mr Lam, who was injured in the attack, said he was angry about a slow police response after he alerted them to the trouble, government-funded broadcaster RTHK reported. Mr Lam said the police action had failed to protect the public, allowing the triads to run rampant. "Is Hong Kong now allowing triads to do what they want, beating up people on the street with weapons?" he asked reporters. Pro-democracy lawmaker Claudia Mo said there was “more than apparent” involvement from the triad, a branch of organised crime in Hong Kong. “What happened last night doesn’t seem accidental in any way,” Ms Mo said. “It’s all organised.” Hong Kong’s leader Carrie Lam described the train station attack as "shocking" but said allegations that police colluded with the assailants were “unfounded”, adding that authorities would investigate fully. She also strongly condemned some anti-government protesters for an "attack" on China's main representative office in the city. Earlier on Sunday, police fired rubber bullets and tear gas to disperse activists after they had fled China's Liaison Office, its main representative branch in the city, after defacing some walls and a national emblem. The action came after more than 100,000 people marched through the city to demand democracy and an investigation into the use of force by police to disperse crowds at earlier protests. Ms Lam said the targeting of the office was a "challenge" to national sovereignty, condemning violent behaviour of any kind. She said throwing eggs and black paint on the emblem “hurt the nation’s feelings.” The Chinese government has condemned the action, with the country’s Foreign Ministry saying on Monday that the behaviour of some "radical" Hong Kong protesters violates the bottom line of the "One Country, Two Systems" formula through which Beijing administers the territory. The official <em>People's Daily</em> newspaper said on Monday in a front-page commentary that the protesters' actions were "intolerable." The article headlined “Central Authority Cannot be Challenged” expanded on a strong condemnation issued the previous night by the government’s Hong Kong and Macao Affairs Office. The growing tumult has fuelled fears that China’s People’s Liberation Army may intervene. A Chinese army brigade said on Monday that it held an anti-terrorism exercise in southern Guangdong province. The 74th Army Group did not refer to Hong Kong in its social media statement, but military commentator and retired officer Yue Gang said that troops would be dispatched to the semi-autonomous territory if needed. “To deface the Chinese national emblem is like acting as an enemy to 1.3 billion Chinese people,” Mr Yue said. “They must be deterred.” Hu Xijin, editor-in-chief of the nationalistic <em>Global Times</em>, warned against military intervention. “If the People’s Liberation Army helps to stabilise the situation, Hong Kong will benefit from law and order, but the public opinion won’t buy it,” Mr Hu wrote in a commentary published on Weibo, a popular Chinese microblogging service. The public will “accuse Beijing of undermining ‘one country, two systems,’” he said. The “one country, two systems” framework allows Hong Kong a fair degree of autonomy in local affairs. But two months of sometimes violent protests has produced the most serious crisis since Britain handed the freewheeling city back to Chinese rule in 1997. Demonstrators fear an erosion of rights and freedoms in the territory. Protesters are demanding the full withdrawal of a bill to allow people to be extradited to mainland China for trial, where the courts are controlled by the Communist Party, and independent inquiries into the use of police force against protesters. The continuing unrest also marks the greatest popular challenge to Chinese leader Xi Jinping since he came to power in 2012. The Hospital Authority said 45 people were injured in the Yuen Long attack, with one in a critical condition. Some 13 people were injured after the clashes on Hong Kong island, one seriously, the authority said. In a statement early on Monday, police "strongly condemned" both violent incidents and were investigating both cases. Some police had been injured in the clashes after protesters hurled bricks, smoke grenades and petrol bombs, the statement said.