HONG KONG • Mr Nathan Law, who is one of Hong Kong's most prominent young democracy activists, announced late last night that he had fled abroad in response to Beijing imposing a sweeping security law on the city.

"I have already left Hong Kong and continue the advocacy work on the international level," Mr Law said in a short English message to journalists, declining to say which country he had gone to.

"Based on risk assessment, I shall not reveal too much about my personal whereabouts and situation now," the 26-year-old added.

The revelation that Mr Law had left Hong Kong came as the local government released a statement confirming that a popular protest slogan widely used by demonstrators over the past year was now illegal under Beijing's new law.

"Liberate Hong Kong, Revolution of our Times" has become a clarion call for pro-democracy protesters. It is chanted by huge crowds and plastered on banners.

For some, it represents genuine aspirations to split Hong Kong from China. But for many others, it is more a cry for democracy and expression of rising frustration with Beijing's rule.

Mr Law had shouted the slogan in a video message to a US Congress committee on Wednesday, not long after the law took effect.

In a statement, Hong Kong's government said the phrase was now illegal because it "contains the indications of Hong Kong independence, or alienating Hong Kong from China, or changing its legal status, or subverting the state".

The latest development is seen as confirmation that certain peaceful political views have become illegal since the law came into effect on Tuesday night.

On Wednesday, the first arrests were made under the legislation, mainly of protesters found in possession of pro-independence flags or stickers.

AGENCE FRANCE-PRESSE