Nizar Zakka, a Lebanese a former hostage in Iran, has joined other rights activists protesting at the venue of nuclear talks between Tehran and world powers in Vienna.

Zakka said on Friday that he joined a sit-in that Barry Rosen, a former US diplomat held hostage in Iran for 444 days four decades ago, started earlier this week to demand the release of all foreign hostages in Iran before the United States and other world powers reach any agreement with Iran.

Zakka himself has established an NGO to support and help the release of people held hostage for unjust reasons. Zakka was detained in Iran on vague espionage charges, in what human rights organizations have described as essentially hostage-taking by the Islamic Republic.

UK-based Iranian journalist and activist Jamshid Barzegar has also joined Rosen’s sit-in that is gaining momentum. US Special Envoy for Iran Robert Malley met with Rosen on January 19 and urged him to end his hunger strike.

On Wednesday, a French tourist, Benjamin Briere, also detained in Iran on vague charges appeared in court where the judge made Iran’s intentions clear by using the word prisoner "swap", according to his lawyer.

Iran has detained many foreigners and dual nationals who have visited the country and used them as bargaining chips against Western countries, according to human rights organizations.