Protasevich was detained in Belarus, where the plane he was travelling on was forced to land.
"I simply plead with all the international community... please, world, stand up and help, I beg you so much because they will kill him," said Protasevich's mother Natalia Protasevich.
In a video posted online on Monday (May 24), Protasevich said he was in good health and acknowledged having played a role in organising mass disturbances in Minsk last year, but the comments were dismissed by his allies as made under duress.
"A doctor that looked at the video saw marks of strangulation on his neck...Its quite obvious he is being tortured behind the walls of this prison," Natalia said, adding that there were also signs his teeth and nose had been broken.
Belarus denies it mistreats detainees. Rights groups have documented what they say are hundreds of cases of abuse and forced confessions since last year.