An Italian pro-Palestinian activist has been found dead in the Hamas-governed Gaza Strip hours after being abducted.
Vittorio Arrigoni, 36, was seized on Thursday by a radical group that has been in conflict with Hamas and is seeking the release of its leader.
Police said he was found hanged in a Gaza City house after receiving a tip-off. Two people have been arrested.
Italy denounced the "barbaric murder", calling it an "act of vile and senseless violence".
Mr Arrigoni was the first foreigner kidnapped in Gaza since BBC journalist Alan Johnston was abducted in 2007.
Friends of the activist gathered outside the hospital where his body had been taken on Friday morning.
"He came from across the world, left his country and family and his entire life and came here to break the siege, and we kill him? Why?" asked one of his friends.
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BBC News, Ramallah
Vittorio Arrigoni was a fierce critic of Israel but it is Palestinians that killed him. The 36-year-old was a passionate defender of the rights of people in Gaza and Palestinian friends have opened a mourning tent in Gaza City.
There have been no kidnappings of this kind since Hamas came to power in 2007. The Islamist movement will see this as a challenge to its authority.
Two members of an al-Qaeda inspired Salafist group have been arrested. Salafists practise an ultra-conservative form of Islam and regard Hamas as too moderate.
Many will be watching to see whether Hamas responds with violence. In 2009, Hamas killed over 25 Salafists after one group challenged its leadership.
The movement is treading a fine line. It does not want to alienate conservative Islamists in Gaza but, at the same time, Hamas has had to become increasingly pragmatic, governing for a population living under much hardship.
Italian diplomats have been in touch with Israel regarding the transfer of the body from the Gaza Strip, possibly on Sunday, an Israeli official has told the BBC.
Vittorio Arrigoni was seized by Salafist radicals, an Islamist movement itself that considers Hamas as too moderate, BBC Gaza correspondent Jon Donnison says.
The Salafists had threatened to execute Mr Arrigoni by 1400 GMT on Friday unless several prisoners, including their leader, Sheikh Abu Walid al-Maqdasi, were released. Sheikh Maqdasi was arrested by Hamas police last month in Gaza City.
In a video posted on YouTube, Mr Arrigoni appeared to have been beaten and his eyes were covered with thick black tape.
A caption on the video read: "The Italian hostage entered our land only to spread corruption." The video called Italy "the infidel state".
It is not clear why Mr Arrigoni was killed before the given deadline, but the Hamas interior ministry said he had died soon after being abducted.
Ministry spokesman Ehab al-Ghussein said he was killed "in an awful way".
Vittorio Arrigoni was kidnapped on Thursday morning
Mr Ghussein told reporters that the security forces had been led to the house in Gaza City by one of the men involved.
He described the killing as a "heinous crime which has nothing to do with our values, our religion, our customs and traditions", and vowed to hunt down and bring to justice others who were involved.
In Rome, the Italian foreign ministry expressed "its deep horror over the barbaric murder".
Mr Arrigoni was a member of the International Solidarity Movement (ISM) and had been in Gaza for several years.
Huwaida Arraf, a co-founder of the ISM, said he was very well-known in the territory and had a "dynamic, humanitarian personality".
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Salafists in Gaza
Small in number but appear to be attracting supporters
View Hamas as too moderate
Refuse to abide by ceasefires
Launched hundreds of rockets at Israel
Salafism espouses an austere form of Sunni Islam based on practices of earliest Muslims
"I even thought that whoever has him is going to see his humanity and just let him go, so when I heard what happened to him I was totally shocked," she told the AFP news agency.
Inge Neefs of ISM Gaza says that there are now four ISM foreign activists in the Gaza Strip.
Ms Neefs told the BBC that they intended to stay as most Palestinians do not support the extremists who kidnapped Mr Arrigoni and they should not be blamed for it.
Hamas had been credited with eliminating the threat of kidnapping in Gaza until his abduction.
Israeli soldier Gilad Shalit has been held in Gaza since June 2006. He was captured in a raid into Israel by Hamas and other militant groups.
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