The British citizen, facing court actions that could see him jailed and liable to pay compensation of 300 million baht, said he was also ''very disappointed'' with the lack of support from his embassy.
The latest accusation over two controversial videos posted on YouTube means he will try to meet tomorrow with European Union officials to update them on the case.
The EU is at present negotiating a free trade agreement with Thailand, and the issue of workers' rights in Thailand is being looked at closely.
A large Thai company, Natural Fruit Ltd, has accused Mr Hall, 33, of criminal defamation and breaking the Computer Crime Act for disseminating his research alleging violations of migrant labor rights in its factory.
This week notification of a fresh claim was sent to Mr Hall in an email from the British embassy - but he was told that he would have to deal directly with the police to find out more.
As a result, Mr Hall went to Bangkok's Bangna Police Station last night to examine the new allegations.
''After last night's experience, I am very uncomfortable with the justice system here,'' Mr Hall said today. ''The document was a confession and I wasn't prepared to sign it.
''How can I get a fair trial? I have no confidence of that happening now.''
Mr Hall said he had seen migrants treated poorly by police and was surprised to experience breaches of human rights himself for the first time.
Although he reads and speaks Thai reasonably well, Mr Hall said he was not provided with an English translation of the document presented to him last night.
''My basic rights were denied,'' he said today. ''I had no right to a translator and no right to comment.''
He refused to sign because his knowledge of Thai script enabled him to see that the document was described as a confession, not an acknowledgement of the charge.
Mr Hall said that it was ''a very strange style of work.'' He said he believed the translator provided by police was ''someone pulled off the street.''
If at some point Mr Hall is arrested, he says he will not seek bail and will go to jail before fighting his case in court. ''I am a human rights defender, not a criminal,'' Mr Hall said.
The new allegations against him concern two videos posted online by Finnwatch, the rights group that commissioned his original research, and Aljazeera, a prominent international news outlet.
He said that although the complaint dated from July, one of the videos was only posted online this month.
''I still don't understand what the charge was,'' he said. The officer who was dealing with him appeared to be on the telephone to others almost constantly.
''Justice was not present in that police station last night,'' he said. ''The way I was dealt with was simply chaotic and completely untransparent.''
His legal research team continues to fear that an arrest warrant could be issued without proper process.
Mr Hall has been living in Burma but returned to Thailand briefly on Friday for a health checkup.