|Al Jazeera's Nicole Johnston reports on Arrigoni's abduction and subsequent brutal killing in Gaza
Hamas officials have said they found the body of an Italian peace activist based in Gaza who was kidnapped by armed assailants.
The officials said Hamas police stormed an apartment in Gaza Strip belonging to a member of the group that released a video of the activist, identified as Vittorio Arrigoni.
Mahmoud Abbas, the Palestinian president, strongly condemned the murder as he conveyed his condolences to Arrigoni's family.
"This heinous crime is far from the traditions of our people, who are struggling for freedom and independence," he said.
Abbas added that the role of the International Solidarity Movement (ISM) - which Arrigoni was a part of - in defending Palestinian rights was highly valued.
Earlier in a statement, the Hamas interior ministry said the man was killed shortly after he was abducted at midday on Thursday.
Ehab al-Ghussein, ministry spokesman, said he was killed "in an awful way." He said the kidnappers planned from the beginning to kill him, not to trade him for captives.
A ministry statement denounced the killing as "a crime that does not reflect the values, morals, religion and customs" of the people of Gaza.
An Italian doctor was on his way from Israel to examine the body, a Hamas official said.
In Rome, the Italian foreign ministry condemned what it called a "barbaric murder" and a "vile and irrational gesture of violence on the part of extremists indifferent to the value of a human life."
After a clash early on Friday hours of his abduction, Hamas police found the body of the peace campaigner. It was not immediately clear how he died.
Al Jazeera's Nicole Johnston, reporting from Gaza, said: "Hamas has rounded up a lot of Salafi members since this news came out to try and get more information but at this stage they have told us that there has only been one confirmed arrest."
"Hamas is very keen to say that the security situation in Gaza is solid. It's been really pushing home the message that Hamas is in control and that Gaza is safe," she said.
Foreign aid workers said Arrigoni was an activist with the pro-Palestinian ISM, and also worked as a journalist and writer.
Release of Salafi prisoners
In a video posted on YouTube on Thursday, the group said it had taken him hostage in order to secure the release of an unspecified number of their members who had been arrested by the security forces in the Hamas-ruled Gaza Strip.
It said it would execute him if their demands were not met by 5:00 pm (1400 GMT) on Friday.
The Al Jazeera correspondent said: "The little that we do know about why he was kidnapped in Gaza has come from Youtube video that those responsible posted online."
"In that video they said they would not release him until all of Salafi prisoners who are currently in jail in Gaza, are freed. They also called for release of one of the main Salafi leaders. They said they would not free him until those people were freed from jail. That seems to be the motive behind the case."
"We kidnapped the Italian prisoner Vittorio and we call on the Haniya government ... to release all our prisoners," it said, referring to Hamas premier Ismail Haniya.
"If you don't respond quickly to our demands, within 30 hours from 11:00 am (0800 GMT) on April 14, we will execute this prisoner," it said.
"We don't know which group was behind it. At this stage the main Salafi groups are denying any responsibility. So the situation is quite confusing as who is exactly behind it," the Al Jazeera correspondent said.
Arrigoni was shown blindfolded in the 3-minute clip with blood around his right eye and a hand can be seen pulling his head up by his hair to face the camera.
The Arabic text that accompanied the footage of Arrigoni also said "the Italian hostage entered our land only to spread corruption" and it described Italy as "the infidel state".
Arrigoni is the first foreign national to be abducted in the Gaza Strip since BBC journalist Alan Johnston, who was held for 114 days by a group named the Army of Islam. He was released in 2007.