China is pressing countries to boycott a British-organised event at the United Nations in Geneva on media freedom in Hong Kong with the son of a jailed media tycoon, a letter showed and four diplomats confirmed on Tuesday.

The event on Wednesday titled ‘Media Freedom in Hong Kong’ is being held on the sidelines of the five-week meeting of the U.N. Human Rights Council. Among the speakers is Sebastien Lai, the son of Jimmy Lai who this week marked his 1,000th day in a Hong Kong prison on charges related to the former British colony’s national security law and sedition.

In a letter circulated widely among diplomats at the U.N. in Geneva, China’s mission asked countries “to refrain from participating in this event in any way”.

“Hong Kong-related issues are China’s internal affairs that brook no external interference,” said the diplomatic note reviewed by Reuters.

Four diplomats confirmed having received it and three of them said that Chinese diplomats had also been contacting some countries individually to not attend or support the event in any way. The diplomats declined to be identified because of the sensitivity of the matter.

Still, so far, at least 22 countries have co-sponsored the event, a document showed, including the United States, France and Germany.

China’s diplomatic mission in Geneva did not respond to Reuters request for comment. Hong Kong authorities released a general statement saying it condemned “misleading and slanderous remarks” by organisations and media in the context of the Lai case which amounted to political interference.

Britain’s Foreign, Commonwealth and Development Office said it would continue to raise “longstanding and legitimate concerns over the erosion of rights and freedoms in Hong Kong,” citing the Sino-British Joint Declaration which is meant to guarantee Hong Kong’s autonomy.

The financial hub returned to China from Britain in 1997. Diplomatic tensions between Chinese authorities and Britain have been running high in recent years.

Britain released a report this month saying that Hong Kong authorities have extended the application of a Beijing imposed national security law “beyond genuine national security concerns”. Jimmy Lai’s trial under the new law has been postponed to Dec. 18 and is expected to last 80 days.

China, one of the 47 members of the Geneva-based Human Rights Council and seeking re-election next month, has in the past sought to counter criticism of its human rights record.

Last year, it asked the then-U.N. human rights chief Michelle Bachelet not to publish a highly-anticipated report which said that China’s arbitrary and discriminatory detention of Uyghurs in Xinjiang may constitute crimes against humanity.