By Mohammed Assadi

RAMALLAH, West Bank, June 6 (Reuters) - An estimated 616 Palestinians have been killed in factional fighting since Hamas defeated Fatah in elections in January 2006, a leading Palestinian rights group said on Wednesday.

President Mahmoud Abbas of Fatah said earlier this week that Palestinians were at the brink of civil war and the danger posed by factional fighting was equal to and sometimes exceeded the "danger of occupation" by Israel.

In its annual report, the Palestinian Independent Commission for Citizens' Rights said 345 Palestinians were killed in factional fighting in 2006.

In the first five months of 2007, another 271 Palestinians were killed in factional fighting, the commission said.

A Reuters count puts the number of Palestinians killed by Israelis since January 2006 at 659.

Fighting between Fatah and ruling Hamas Islamists surged late last year and continued into 2007, despite the formation of a Palestinian unity government in March.

The bulk of fighting was in Gaza, the power base of Hamas. Israel withdrew from Gaza in 2005.

Last month some 50 Palestinians were killed in factional fighting. Despite the latest ceasefire deal, Abbas's Presidential Guard accused Hamas fighters on Tuesday of staging an early morning assault on a Fatah base near the strategic Karni commercial crossing in Gaza.

"The report is very tragic," said Mamdouh al-Aker, the commissioner for the human rights group.

Copies of the Palestinian Independent Commission for Citizens' Rights report were sent to Abbas, who formed a committee to look at the findings.

The commission also reported an increase in the number of Palestinians tortured by members of the Palestinian security services.

The commission reported 133 torture cases in 2006 and said two security officers were dismissed for abuse. The commission did not provide torture figures for the first five months of 2007.

Israeli human rights groups said in a report in May that Israeli security interrogators routinely mistreat and sometimes torture Palestinian detainees.

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