Russia defended China's law on Hong Kong slamming "external interference".

Speaking at a video conference in Moscow, Foreign Ministry spokeswoman Maria Zakharova called the situation in Hong Kong "a purely internal affair" of China.

"If we all respect the sovereignty and territorial integrity of the People's Republic of China, and I think there is no doubt about that, then we should consider the situation in Hong Kong as a purely internal matter of China. This also implies our Russian position: we are categorically against external interference in relations between the central government and one of the country's regions," she said.

On Tuesday, a new National Security Law was unanimously passed by China's National People's Congress and then signed by President Xi Jinping.

The law, which makes it a crime to undermine Beijing's authority in Hong Kong, came in the wake of months of protests last year in the semi-autonomous territory.

Hong Kong, a special administrative region of China, witnessed large protests last year against a move to legalise extradition of accused people to mainland China.

The new law, comprising six chapters and 66 articles, criminalises four types of acts: secession, subversion of state power, terrorist activities, and collusion with foreign or external forces to endanger national security.

The new legislation was met by harsh criticism in Europe.

On Wednesday morning, over 300 Hong Kong activists were arrested for demonstrating against the new law and raising pro-independence flags and banners.