On Monday morning, 13-year-old Ibrahim Ahmad, devoutly religious as well as a keen sports fan, went to dawn prayers at the local mosque before taking a taxi with his three older brothers to the neighbouring town of Beit Lahiya and joining the steadily growing procession on foot to the Yasser Arafat commemoration in central Gaza city.

By the afternoon, he was in the morgue in Shifa hospital, shot in the neck and side, the youngest of the seven victims of the bullets fired by Hamas forces in the bloody aftermath of the rally.

Ibrahim's brother Amjad, 23, explained that they were separated during the rally but thought little of it because of the happy, almost carnival, atmosphere of the occasion. "We were happy because there were so many people," he said. "There was no one left in Beit Hanoun. About 6,000 went."

Even after they heard the shots as the rally started to disperse, they didn't imagine anything could have happened to Ibrahim. "We waited for him to get back home but when he didn't come, we went to look for him and found him in the hospital."

As the mourners sat on a long row of plastic chairs in the alley next to Ibrahim's home, his father was too distraught to join in the increasingly political family discussion raging round him about the meaning of Monday's events. "He went to the rally and he did not come back," he said. "He's a kid. He had no weapons or anything else. I blame the security forces. They're the ones supposed be in charge of security."

He meant the uniformed Hamas men who had opened fire. But other relatives were less reticent. "Even the Israelis do not do this," said Ibrahim's uncle, Jihad Auda, 52, adding of the rally: "It was like a party, a wedding. They know that these weapons will kill. Why do they use it against their own people?"

Mr Auda was also highly sceptical of the Hamas explanation that its force had been first fired on by Fatah gunmen posted at the nearby al Azhar university – denied both by the rally's organisers, officials of the Fatah-linked university, and students who had been on the campus at the time. Even if it were true, he said "they should react by shooting at the people who are shooting them, not by shooting at the masses".

The Ahmads are a staunchly Fatah family, and the mourning relatives said the shootings would add to what they insisted was Hamas's growing unpopularity because of its failure to serve "the interests of the people". Mr Ahmad lost his job like tens of thousands of others when Israel shut down the Karni cargo crossing in response to Hamas's seizure of control in Gaza in June. "The crossings are closed and now these killings happen," he said.

Fatah claimed Hamas, which continued yesterday publicly to blame its rival faction for triggering the violence at Monday's unexpectedly huge commemoration, had detained 450 of its supporters. Hamas denied the people had been held for supporting Fatah, but for incitement to violence at the rally.

After a meeting of the Hamas de facto government in Gaza to discuss the violence, Taher Nunu, its official spokesman said: "The government will reconsider its policy toward Fatah and will take additional steps to protect the political and national and media movements in the streets of Gaza."

Some local observers interpreted the statement as raising the prospect of even tougher measures against political opponents.

Meanwhile the Palestinian Centre for Human Rights, which has a long record of criticising abuses by Israeli forces and both the main Palestinian factions, issued a strongly worded condemnation of "the excessive use of force" by the Hamas police at the rally. Its investigation into the rally said that, at about 11.30am, a jeep carrying plain-clothes Hamas gunmen drove "provocatively" in the direction of hundreds of demonstrators.

The report added of the subsequent violence: "The demonstrators threw rocks at the police who responded by indiscriminate gunfire and raiding the rally grounds. Pictures showed members of the police deployed around [the grounds] firing indiscriminately at the rally participants."

It questioned the claims by Hamas that its police or security force members had been injured saying they had not found such a case. The document added: "The duty of the police and security personnel is to protect participants. And in the case of coming under gunfire, as claimed by governmental sources in Gaza, indiscriminate, excessive, and lethal firing by the police at civilian gatherings is not justified."