Hundreds of mourners have rallied and many have held a candlelight vigil in the Hamas-governed Palestinian enclave of Gaza for Vittorio Arrigoni, an Italian activist who was killed on Friday.
And in the West Bank, which is run by Fatah, Hamas's rival, around 100 people, most of them foreigners, marched on Saturday through Ramallah to a house of mourning in El Bireh, an AFP correspondent said.
Vittorio Arrigoni, 36, who was working with the pro-Palestinian International Solidarity Movement (ISM), was found dead by the security forces in a house in northern Gaza early on Friday.
He had been hanged, Hamas security officials said.
Hamas officials said two people had been arrested on suspicion of involvement in the kidnapping and said they were hunting further accomplices.
Ihab al-Ghussein, a Hamas spokesman, called it a "heinous crime which has nothing to do with our values, our religion, our customs and traditions".
"The other members of the group will be hunted down," he said.
There has been an outrage over the cold-blooded killing of the Italian.
"I was about to cry when I heard the news. That man quit his family for us, for Gaza, and now Gazans killed him. That was so bad," Abu Ahmed, a supermarket owner, said.
Samira Ali, a teacher, said: "Those who killed him are not Muslims and certainly not Palestinians."
Arrigoni's kidnappers described him as a "journalist who came to our country for nothing but to corrupt people" - a charge completely rejected by activists and aid workers who knew him in Gaza.
"He's very well-known, he lives among the people," said Huwaida Arraf, a co-founder of ISM. "Vit has repeatedly put his life in danger, put his life on the line in support of the Palestinians."
A journalist colleague at the Italian daily Il Manifesto said he was "astounded" by Arrigoni's death.
Arrigoni is the third ISM member to be killed in Gaza - US national Rachel Corrie was crushed to death by an Israeli bulldozer in March 2003, and a month later Briton Tom Hurndall was shot and critically injured by the army. He died in January 2004.
The murder drew widespread condemnation, from Gaza City to Italy to the UN.
Italy's foreign ministry expressed "deep horror over the barbaric murder," saying it was an "act of vile and senseless violence committed by extremists who are indifferent to the value of human life."
Ban Ki-moon, the UN secretary-general, called for "the perpetrators of this appalling crime to be brought to justice as soon as possible", his spokesperson said in a statement.
Arrigoni was kidnapped a day earlier by a group aligned with al-Qaeda which had demanded that Hamas release Salafist prisoners within a 30-hour deadline that was to have expired on Friday afternoon. It was not clear why they killed him.
In a video posted on YouTube, the kidnappers said Arrigoni had been taken hostage in order to secure the release of an unspecified number of Salafists detained by Hamas.
Arrigoni had been in the Palestinian territories for 10 years, first in the West Bank. He was asked to leave by Israel and he arrived in Gaza in August 2008 with the first ship of the Gaza Free Movement.
There are half a dozen radical Islamist groups in Gaza, with membership numbering in the hundreds. The differences between them are unclear.
Some analysts believe they work in cells to evade Hamas pressure. All want to end Western influence and establish an Islamic state across the Middle East.
Hamas forces in August 2009 killed 28 people, mostly Salafis, in the storming of a mosque where a religious leader who supported al-Qaeda surrounded himself with armed men and declared an Islamic emirate.