Authorities in Qatar have barred a prominent human rights lawyer and former justice minister from leaving the country, a rights group said on Friday. 

Najeeb al-Nuaimi was also a lead defence lawyer in ousted Iraqi president Saddam Hussein's trial, and has since voluntarily defended prisoners of conscience in the Gulf state.

"Authorities in Qatar prevented... Nuaimi from travelling without informing him about any possible reasons," the Gulf Centre for Human Rights said in a statement.

It said he had been placed on a list of individuals banned from leaving the state, without giving "prior clarification or directing any charges against him".

One of Nuaimi's clients, poet Mohamed Rashid al-Ajami, was given a life sentence in 2011 for a poem he wrote criticising the Gulf state's ruler, which authorities said incited violence against the state.

Ajami was released in 2016 after serving more than four years in jail.

Nuaimi was also head of an international committee in 2003 to defend suspected Islamist militants imprisoned by the US military in Guantanamo.

Amnesty International condemned the decision to bar him from leaving the country.

In a statement Friday, it urged Qatar "to immediately rescind the ban or to urgently provide to him, in writing, its legitimate legal basis and reasons for doing so".

Qatar, like most Gulf states, has been spared much of the unrest brought in by the Arab Spring uprisings that have rocked the Middle East since 2011.

But activists throughout the region have resorted to social media platforms such as Twitter, Facebook and YouTube to advocate human rights.

That has prompted what rights groups say is a systematic clampdown by authorities.