Sitting on the floor, wearing traditional Islamic clothes and holding an old notebook, Abu Hamizi, 22, spends at least six hours a day searching internet chatrooms linked to gay websites. He is not looking for new friends, but for victims.

"It is the easiest way to find those people who are destroying Islam and who want to dirty the reputation we took centuries to build up," he said. When he finds them, Hamizi arranges for them to be attacked and sometimes killed.

Hamizi, a computer science graduate, is at the cutting edge of a new wave of violence against gay men in Iraq. Made up of hardline extremists, Hamizi's group and others like it are believed to be responsible for the deaths of more than 130 gay Iraqi men since the beginning of the year alone.

The deputy leader of the group, which is based in Baghdad, explained its campaign using a stream of homophobic invective. "Animals deserve more pity than the dirty people who practise such sexual depraved acts," he told the Observer. "We make sure they know why they are being held and give them the chance to ask God's forgiveness before they are killed."

The violence against Iraqi gays is a key test of the government's ability to protect vulnerable minority groups after the Americans have gone.

Dr Toby Dodge, of London University's Queen Mary College, believes that the violence may be a consequence of the success of the government of Nouri al-Maliki. "Militia groups whose raison d'être was security in their communities are seeing that function now fulfilled by the police. So their focus has shifted to the moral and cultural sphere, reverting to classic Islamist tactics of policing moral boundaries," Dodge said.

Homosexuality was not criminalised under Saddam Hussein – indeed Iraq in the 1960s and 1970s was known for its relatively liberated gay scene. Violence against gays started in the aftermath of the invasion in 2003. Since 2004, according to Ali Hali, chairman of the Iraqi LGBT (Lesbian, Gay, Bisexual, Transgender) group, a London-based human-rights group, a total of 680 have died in Iraq, with at least 70 of those in the past five months. The group believes the figures may be higher, as most cases involving married men are not reported. Seven victims were women. According to Hali, Iraq has become "the worst place for homosexuals on Earth".

The killings are brutal, with victims ritually tortured. Azhar al-Saeed's son was one. "He didn't follow what Islamic doctrine tells but he was a good son," she said. "Three days after his kidnapping, I found a note on my door with blood spread over it and a message saying it was my son's purified blood and telling me where to find his body."

She went with police to find her son's remains. "We found his body with signs of torture, his anus filled with glue and without his genitals," she said. "I will carry this image with me until my dying day."

Police officers interviewed by the Observer said the killings were not aimed at gays but were isolated remnants of the sectarian violence that racked the country between 2005 and 2006. Hamizi's group, however, boasts that two people a day are chosen to be "investigated" in Baghdad. The group claims that local tribes are involved in homophobic attacks, choosing members to hunt down the victims. In some areas, a list of names is posted at restaurants and food shops.

The roommate of Haydar, 26, was kidnapped and killed three months ago in Baghdad. After Haydar contacted the last person his friend had been chatting with on the net, he found a letter on his front door alerting him "about the dangers of behaving against Islamic rules". Haydar plans to flee to Amman, the Jordanian capital. "I have… to run away before I suffer the same fate," he said.

According to Human Rights Watch, the Shia militia known as the Mahdi army may be among the militants implicated in the violence, particularly in the northern part of Baghdad known as Sadr City. There are reports that Mahdi army militias are harassing young men simply for wearing "western fashions".

A Ministry of Interior spokesperson, Abdul-Karim Khalaf, denied allegations of police collaboration. "The Iraqi police exists to protect all Iraqis, whatever their sexual persuasion," he said.

Hashim, another victim of violence by extremists, was attacked on Abu Nawas Street. Famous for its restaurants and bars, the street has become a symbol of the relative progress made in Baghdad. But it was where Hashim was set on by four men, had a finger cut off and was badly beaten. His assailants left a note warning that he had one month to marry and have "a traditional life" or die.

"Since that day I have not left my home. I'm too scared and don't have money to run away," Hashim said.