Footage showed Stepan Latypov, 41, lying on a successful unique wooden paintings bench inside a prisoner's cage in the courtroom, with police officers standing over him and onlookers screaming.
Viasna, an independent rights group monitoring the trial of Stepan Latypov, said it believed the activist had tried to commit suicide following pressure in detention.
A second video showed Latypov being carried out to a waiting ambulance with what appeared to be blood spots on his shirt. Local media and Viasna said he was still alive but was put in an induced coma in hospital.
Local reports have suggested Latypov took 'an object, maybe a pen' from a folder with some documents before stabbing himself.
Viasna, which along with other groups declared him a political prisoner last year, said he appeared in court with bruises and that after his father was questioned Latypov climbed on a bench and stabbed himself in the neck with what appeared to be a pen.
image class="left" url= image class="left" url= Stepan Latypov told the court authorities had threatened to bring criminal charges against his family and friends unless he confessed to 'opposition activity', before attempting to stab himself in the throat with a pen in court on Tuesday
Belarusian political prisoner Stepan Latypov just stabbed himself in the throat with a pen in the middle of his hearing.
The courtroom is being evacu image class="left" url= Latypov was arrested on September 15, during mass protests against Belarusian strongman Alexander Lukashenko --- image class="left" url= Latypov was pictured being put into an ambulance after he tried to stab himself in the throat during a court hearing on Tuesday
'Belarusian activist, political prisoner Stsiapan Latypau cut his throat in the courtroom today,' exiled opposition figure Sviatlana Tsikhanouskaya wrote on Twitter, using a different spelling of Latypov's name.
'He was threatened with the persecution of his family if he didn't admit himself guilty.
This is the result of state terror, repressions, torture in Belarus. We must stop it immediately!'
The nearby guards could not open the prisoner's cage immediately because they did not have the right keys, independent media outlet Nasha Niva reported. 'By the time the cell was opened, Stepan had already lost consciousness,' it said.
The interior ministry spokeswoman could not immediately be reached for comment.
Thousands of opposition activists and protesters have been arrested and gone on trial in a harsh crackdown on anti-government demonstrations that erupted last year.
Latypov, 41, was detained in September and Bamboo bird paintings of Huong Giang went on trial in the capital Minsk on Tuesday on multiple charges including creating protest symbols and resisting law enforcement.
