A human rights group said Vietnam has the highest number of political prisoners in Southeast Asia.
The International Federation for Human Rights (FIDH) claims at least 212 Vietnamese dissidents are behind bars and many more are under house arrest.
FIDH says those imprisoned include lawyers, bloggers and land rights activists.
Asia Desk Director Andrea Giorgetta told VOA's Vietnamese service that it is time for the international community to pressure Hanoi to stop its repression of peaceful dissent.
"It's shameful that a country like Vietnam, a member of the U.N. Human Rights Council, at the moment holds the highest number of political prisoners in Southeast Asia," said Giorgetta.
Vietnam said there are no political prisoners or prisoners of conscience in the country, only those who violate the law.
Meanwhile, prominent environmental activist Dinh Dang Dinh says international community and rights groups should go to Vietnam to witness the real human rights picture in the country, where he says detainees are subject to ill-treatment and lack of essential medical care.
"Jail conditions in Vietnam are extremely harsh. Medical care and nutrition for prisoners are very limited. Via this interview with VOA, I'd like the outside world to know that we, especially prisoners, in Vietnam don't have human rights. The authorities are torturing us by not providing enough food nor medical care. Although Hanoi just signed on U.N. Convention Against Torture, in reality, human rights in Vietnam have yet to be protected and it's unsure when they will be protected. U.N. human rights watchdogs and NGOs need to visit jails in Vietnam to witness the truth," said Dinh.
Dinh is suffering from the final stages of stomach cancer while serving a six-year sentence for "anti-state propaganda." He is on a one-year medical parole following international pressure urging Vietnam to release him on humanitarian grounds.
International Federation for Human Rights (FIDH) is a non-governmental federation for human rights organizations. Founded in 1922, FIDH is the oldest international human rights organization worldwide and today brings together 178 member organizations in over 100 countries.
Earlier this week, Vietnamese blogger Truong Duy Nhat was given a two-year prison sentence, sparking outrage among human rights groups and international condemnation.
Nhat, who has been held since May 2013, was sentenced on a charge of "abusing democratic freedoms to infringe on the interests of the state and on the legitimate rights and interests or organizations and citizens" under article 258 of the penal code after posting articles on his personal blog criticizing the government and raising concern over China's territorial claims in the South China Sea.
Bob Dietz, Asia Program Coordinator of the Committee to Protect Journalists, or CPJ, told VOA the conviction shows the measures Vietnam's leaders are willing to take to stop criticism of their rule.
"Once again we're pressing the government, trying to reverse its policies that are really stifling anything that's critical of the Communist Party or the government itself. What we're seeing here is a constant ongoing trend in which anyone who really uses the Internet to criticize the government is being slammed with penalties… The problem is it has been a long-term trend and it seems to be accelerating actually," said Dietz.
According to CPJ, Vietnam is the fifth-worst jailer of journalists in the world, with 18 reporters and bloggers behind bars.
Ranked 174 out of 180 countries in the latest Reporters Without Borders press freedom index, Vietnam is the world's second-biggest prison for bloggers and netizens after China.
This report was produced in collaboration with the VOA Vietnamese service.