Hamas security forces moved swiftly against their Fatah rivals in the aftermath of a mass Fatah rally that ended with seven people dead, rounding up 400 people in an overnight crackdown, Fatah officials said Tuesday.

The detainees included dozens of the rally's organizers, Fatah spokesman Hazem Abu Shanab said. Hamas officials were not immediately available for comment.

Monday's rally, a memorial service for the late Palestinian leader Yasser Arafat, drew 250,000 people, making it Fatah's biggest show of force in Gaza since Hamas took control of the coastal territory in June. It ended in the worst violence Gaza has seen since the Hamas takeover, with seven civilians killed and 85 wounded as Hamas men opened fire on protesters.

Gaza City was quiet and tense Tuesday morning, with few Hamas policemen on the streets. Schools were closed in mourning, and most of the stores in downtown Gaza were shuttered.

Four of the victims were buried Tuesday without incident. On Monday, a funeral for one 19-year-old killed at the rally turned violent when mourners clashed with Hamas men, leaving three people wounded.

In the West Bank, Palestinian President Mahmoud Abbas declared three days of mourning, ordering Palestinian flags to fly at half-staff and condemning the Hamas crackdown as a "heinous crime." Abbas installed a pro-Western government in the West Bank after the Hamas takeover.

Monday's rally in a Gaza City square was seen as a major show of support for the moderate Abbas, Arafat's successor as the leader of Fatah and the bitter rival of Gaza's Hamas rulers. But it quickly descended into mayhem, laying bare the increasing fragility of Hamas' five-month-old hold on Gaza.

In one confrontation, hundreds of young Fatah activists, some wrapped in their movement's yellow flag, faced off against Hamas police in black or blue-camouflage uniforms across an intersection.

The Fatah supporters pelted Hamas troops with stones, surging forward even as they were met by heavy bursts of gunfire. One Hamas policeman dropped to one knee for better aim. At one point, a young stone thrower collapsed and was carried off by others.

At the rally, the crowd cheered and whistled as the movement's remaining leaders — most had fled to the West Bank in June — delivered hopeful speeches about a Fatah comeback.

Former policeman Wasfi Ramadan, 42, who attended with his 14-year-old daughter Salwa and 9-year-old son Yasser, said the rally marked the first time since June that he dared to show his support for Fatah. "I felt that the spirit of Yasser Arafat ... is calling us to rise up and come out," he said.

Tens of thousands of Fatah supporters still draw a monthly salary from Abbas' West Bank government, ensuring loyalty under Hamas rule.

The crowd was starting to disperse when the first shots rang out.

Hamas said it was provoked and that Fatah gunmen, allegedly planted on nearby rooftops, had fired first. The violence, Hamas said, was an attempt to exploit Arafat's memory in order to "cause chaos and confront Hamas."

Fatah denied the charge, saying no armed men were allowed at the rally. A reporter saw 10 Fatah gunmen being turned away at the start of the rally.

After nightfall, Hamas forces began arresting Fatah activists involved in planning the rally, Fatah officials said, and confiscated computers and documents from a Fatah media office.

Fatah's unexpected show of strength Monday came as Abbas prepares for new peace talks with Israel, starting with a U.S.-hosted Mideast conference in Maryland later this month, and tries to fend off Hamas claims that he does not have a mandate to negotiate.

Fatah's former Gaza strongman, Mohammed Dahlan, said Hamas' harsh response Monday was a sign its grip on Gaza is weakening. "What is happening in Gaza today is the beginning of the end of Hamas on the popular, religious and moral level," he told Palestine TV. "There will be victory against the killers soon."

While the international community has welcomed Abbas' new government, Hamas has been widely shunned, and Israeli economic sanctions have hurt an already weak Gaza economy.

The small radical group Islamic Jihad, which has clashed occasionally with Hamas but shares its militant ideology, condemned Hamas for the violence.

"Despite all of the political differences...it's forbidden and taboo to open fire randomly on a mass popular demonstration," said Nafez Azzam, one of the group's leaders in Gaza.