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.