A third anti-government activist has been murdered in Iraq in less than 10 days, police and medics said Wednesday, as the United Nations accused militias of killing and abducting demonstrators.

Protesters have complained of an intensifying campaign of intimidation in a country where pro-Iranian armed groups integrated into the security forces wield growing influence.

The body of 49-year-old father of five Ali al-Lami was found overnight with gunshot wounds to the head, according to his friends, who said he had arrived in Baghdad just days earlier to join the protests.

"It was the militias of a corrupt government that killed him," said a close friend, Tayssir al-Atabi.

A police source said the attackers had used guns with silencers, while forensic experts said Lami had been struck by three bullets.

Iraq's capital and its Shiite-majority south have been gripped by more than two months of anti-government demonstrations in which more than 450 people have died and 25,000 have been wounded.

Despite the threats and violence, protesters massed in Baghdad and the south again on Wednesday calling for the "fall of the regime". The UN urged the Iraqi authorities to hold to account the perpetrators of a string of murders and abductions of activists and protesters.

"Groups referred to as 'militia', 'unknown third parties', 'armed entities', 'outlaws' and 'spoilers' are responsible for the deliberate killings and abductions of demonstrators," said a UN report released Wednesday.

"These acts contribute to a climate of anger and fear. The government must identify those groups responsible without delay and hold perpetrators accountable."

The role of the Hashed al-Shaabi, a network of armed groups integrated into the state, has come under increased scrutiny.