Javid-Tehrani, 29, was first arrested during the 1999 student protests. Human Rights Watch said he should be released to ensure that he has access to adequate medical treatment.
Javid-Tehrani, who has been continually detained since 2005, is on the 18th day of his hunger strike and suffers from health problems related to his torture, the group said in a statement.
He is currently held in the "doghouse" section of the Gohar Dasht prison in Karaj city.
"Behrooz Javid-Tehrani, who is critically ill, is in prison for his peaceful political dissent and his human rights advocacy," said Sarah Leah Whitson, Middle East director at Human Rights Watch. "He should never have been imprisoned in the first place and he should be released at once or he will likely die in prison."
Javid-Tehrani has never had access to a lawyer, and has had limited access to his family since his 2005 arrest, Human Rights Watch said.