Hong Kong‘s pro-democracy protesters are close to creating a “very dangerous situation”, the city’s leader Carrie Lam warned on Monday as train travel and international flights in the global financial hub were thrown into chaos.

The peak-hour efforts to lock down Hong Kong’s transport sector on Monday morning followed two months of unprecedented and often violent unrest, fuelled by demands for greater democracy in the self-governing territory.

Several commuter train lines came to a stop on Monday, when protesters blocked entrance to the trains. Some streets were also taken over by protesters demanding more freedom in the city.

On Sunday, the city government had also said that violence and illegal protests were pushing the city to an “extremely dangerous edge”, with police firing multiple rounds of tear gas to disperse hundreds of anti-government protesters.

The protests have become the most serious political crisis in Hong Kong since it returned to Chinese rule 22 years ago after being governed by Britain since 1842. They have also presented the biggest popular challenge to Chinese leader Xi Jinping in his seven years in power.

China’s official news agency Xinhua said on Sunday: “The central government will not sit idly by and let this situation continue. We firmly believe that Hong Kong will be able to overcome the difficulties and challenges ahead.”

On Sunday night, protesters split into several different directions to disrupt transport networks. Police said they were “seriously paralysing traffic and affecting emergency services” and warned them to stop immediately – an order the protesters ignored.