“There were dozens of them, armed with guns, and they poured into the park,” Ali al-Azawi, 21, the engineering student who had organised the gathering in Basra, said.

“They started shouting at us that we were immoral, that we were meeting boys and girls together and playing music and that this was against Islam.

“They began shooting in the air and people screamed. Then, with one order, they began beating us with their sticks and rifle butts.” Two students were said to have been killed.

Standing over them as the blows rained down was the man who gave the order, dressed in dark clerical garb and wearing a black turban. Ali recognised him immediately as a follower of Hojatoleslam Moqtada al-Sadr, the radical Shia cleric. Ali realised then that the armed men were members of Hojatoleslam al-Sadr’s Mehdi Army, a private militia that fought American forces last year and is now enforcing its own firebrand version of Islam.

The picnic had run foul of the Islamist powers that increasingly hold sway in the fly-blown southern city, where religious militias rule the streets, forcing women to don the veil and closing down shops that sell alcohol or music.

In the election in January, the battle between secular and religious forces in Basra came down to the ballot box. The main Shia alliance triumphed with 70 per cent of the province’s vote, most of the rest going to a secular rival.

That victory has brought to a head the issue of whether Iraq’s new constitution will adopt Islamic law — or Sharia — as most religious Shia leaders desire.

In Basra, however, Islamic militias already are beginning to apply their own version of that law, without authority from above or any challenge from the police.

Students say that there was nothing spontaneous about the attack. Police were guarding the picnic in the park, as is customary at any large public gathering, but allowed the armed men in without any resistance.

One brought a video camera to record the sinful spectacle of the picnic, footage of which was later released to the public as a warning to others.

It showed images of one girl struggling as a gunman ripped her blouse off, leaving her half-naked. “We will send these pictures to your parents so they can see how you were dancing naked with men,” a gunman told her. Two students who went to her aid were shot — one in the leg, the other twice in the stomach. The latter was said to have died of his injuries. Fellow students say that the girl later committed suicide. Another girl who was severely beaten around the head lost her sight.

Far from disavowing the attack, senior al-Sadr loyalists said that they had a duty to stop the students’ “dancing, sexy dress and corruption”.

“We beat them because we are authorised by Allah to do so and that is our duty,” Sheik Ahmed al-Basri said after the attack. “It is we who should deal with such disobedience and not the police.”

After escaping with two students, Ali reached a police station and asked for help. “What do you expect me to do about it?” a uniformed officer asked.

Ali went to the British military base at al-Maakal and pleaded with the duty officer at the gate. “You’re a sovereign country now. We can’t help. You have to go to the Iraqi authorities,” the soldier replied.

When the students tried to organise demonstrations, they were broken up by the Mehdi Army. Later the university was surrounded by militiamen, who distributed leaflets threatening to mortar the campus if they did not call off the protests.

When the militia began to set up checkpoints and arrest students, Ali fled to Baghdad.

A British spokesman said that troops were unable to intervene unless asked to by the Iraqi authorities.

Colonel Kareem al-Zeidy, Basra’s police chief, pleaded helplessness. “What can I do? There is no government, no one to give us authority,” he said. “The political parties are the most powerful force in Basra right now.”

The students have begun an indefinite strike, but fear that there is little that they can do to stop the march of violent fundamentalism. Saleh, 21, another engineering student, said: “If this is how they deal with the most educated in Basra, how would they deal with ordinary people? The soul of our city is at stake.”

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