With four more Iranian human rights defenders arrested in Tehran since August 31, Human Rights Watch (HRW) says the country’s authorities have "ramped up" their crackdown against activists.

"The most reputable rights defenders, who have stepped up to be the messenger for millions of Iranians' discontent, are now in jail," Sarah Leah Whitson, Middle East director at HRW, said in a statement on September 5.

The New York-based watchdog called on the authorities to immediately release the jailed human rights defenders, saying they appear to have been arrested "solely for peaceful dissent."

On August 31, human rights lawyer Mohammad Najafi posted on Facebook that two other lawyers, Farokh Forouzan and Payam Derafshan, were arrested after attending a gathering at the house of another imprisoned lawyer.

Derafshan represented prominent human rights lawyer Nasrin Sotoudeh, who has been detained since June. Her husband, Reza Khandan, was arrested on September 4 after he refused to appear for an interrogation.

In a statement last month, HRW said Sotoudeh's arrest and the new charges against her revealed the "grave degree the Iranian judiciary is criminalizing human rights activism."

Derafshan also represented two other jailed lawyers and the family of Kavous Sayed Emami, a prominent environmentalist who died in detention in February.

Earlier this month, authorities also arrested Hoda Amid and Najmeh Vahedi, two women’s rights activists who are instructors for a workshop on equal rights in marriage.

Amid growing public discontentment in Iran over rising prices and economic hardship, authorities are "only fueling instability with their silencing of peaceful dissent," Whitson said.

"At a time when everyday life is increasingly difficult for millions of Iranians, rights advocates should be an essential part of solving collective problems, instead of a primary target of the government’s crackdown," she also said.