SINGAPORE - Two supporters of law professor Thio Li-Ann have written a letter to two ministries complaining about how she was treated during a recent seminar on human rights.

The letter writers also criticised the European Union delegation to Singapore - which organised the seminar - for having "effectively interfered with the domestic affairs of Singapore" by allowing lesbian, gay, bisexual and transgender (LGBT) activists to protest during Dr Thio's speech at the event.

The letter was sent to the Minister for Social and Family Development Chan Chun Sing and the permanent secretary of the Ministry of Foreign Affairs, Mr Chee Wee Kiong.

A copy of the letter was emailed to the media on Monday.

Dr Thio, a former Nominated Member of Parliament who has spoken out against homosexuality in Singapore, was invited to speak on international human rights law at the EU delegation event on Dec 4.

During her speech, four LGBT activists stood in front of the stage in a silent protest. Their mouths were taped shut with a rainbow-coloured sticker and they held placards with slogans that championed LGBT rights.

In their letter to the ministries,Mr Han Junwei and Ms Sherrie Chong also accused the LGBT activists of having "hijacked" the seminar's question-and-answer session by voicing their views on LGBT issues and making statements against Dr Thio "as an individual".

"This incident is distressing because it evidences a climate where bully tactics, public shaming and mob intimidation are justified under the banner of 'free speech'," they wrote.

"The EU delegation cannot claim to be a neutral arbiter in allowing LGBT protests to take place at a forum on human rights that has no bearing on LGBT issues," they added.

Mr Han and Ms Chong appended to their letter a petition they had started in support of the EU delegation's decision to invite Dr Thio to speak at the seminar.

Their petition, started after LGBT activists had issued a statement last week decrying the same invitation, gathered 1,576 signatures over three days.

A spokesman for the EU delegation to Singapore told The Straits Times that the annual Human Rights Day Seminar was a closed-door event organised by the delegation in consultation with the Singapore authorities. The topic and list of speakers were given to the participants in advance.

"All the panellists who addressed the audience were local. The seminar forms part of the EU's interaction with Singapore's civil society and in no way constitutes interference in the internal affairs of the host country," she said in response to queries from The Straits Times.

She added that the delegation had received petitions both for and against Dr Thio's speech.

The protesters had not been specially invited to the seminar nor had they informed the delegation about the protest in advance, the spokeswoman said.

"Those who initiated the protest were registered participants, in line with the closed-door nature of the event. Given the civil manner in which it was carried out, there were no grounds to intervene or to prevent the protest," she explained.