The previously unknown Swords of Righteousness Brigade, which kidnapped the four men from the Christian Peacemaker Teams (CPT) on 26 November in Baghdad, said the hostages would be killed unless the UK and the US released all prisoners in Iraq.
The four hostages are Norman Kember, 74, of London; Tom Fox, 54, of Clear Brook, Virginia; and Canadians James Loney, 41, and Harmeet Singh Sooden, 32.
The captors' last deadline expired on Saturday.
Aljazeera.net has been inundated with emails and phone calls urging the release of the hostages, adding to the chorus of appeals from prominent Muslim leaders and organisations backed by relatives, friends and well-wishers.
A US citizen wrote to Aljazeera.net saying: "Please let the kidnappers know that there are millions in the West who oppose the US occupation of Iraq and that the four Christian peacemakers are among them.
"Harming the peacemakers will not help advance the cause of justice. We are working to get the US out of Iraq."
A British reader said: "I am so sad that the Iraqi group are holding those who were there to help them. I would like to beg you to please free them. Please let them go.
Echoing the sentiment, a reader in the West Indies wrote: "Know that the world is protesting the kidnapping of the four peacekeepers in Iraq. They are not missionaries, or military men or private contractors but only persons who are seeking to bring about peace."
Other readers noted the hostages' activism in solidarity with Palestinians in the occupied territories.
The hostages were shown in
A message from a New Zealander said: "We are New Zealand citizens and members of Palestine Human Rights Campaign and University Students for Justice in Palestine. ... [Harmeet Sooden] has consistently taken part in public demonstrations against the US/UK invasion and occupation of Iraq."
"He is also a strong supporter of the struggles of the Palestinian people… Harmeet could not possibly be a spy, as alleged by his captors."
Mufti of Palestine
The grand mufti of Palestine, a top religious leader in the occupied territories, said the hostages supported the Palestinian cause.
Shaikh Ikrima Sabri said the four had been visitors to occupied Palestine and had taken part in demonstrations against the illegal barrier being built by Israel cutting off parts of the West Bank.
Shaikh Ikrima Sabri has appealed
for the release of the hostages
"The kidnapping of the four was a surprise to the Palestinian people as they have been supporters of our cause and have stood at our side against the racist separation wall," the mufti said at a news conference.
"It is our duty to support them and to issue a vigorous appeal to the kidnappers to free them."
CPT members along with the hostages' relatives have pleaded for mercy, stressing that their colleagues had been working to restore justice and human rights in Iraq.
"We appeal to your humanity to show mercy on our brothers and let them come back safely to us to continue our work," said CPT, which has offices in Canada and the US.
"As you can see by the statements of support from our friends in Iraq and all over the world, we work for those who are oppressed," it said in a statement to the kidnappers.
"We also condemn our own governments for their actions in Iraq," said the CPT, one of the few remaining aid groups operating in Iraq.
US citizen Tom Fox, 54, is one
of the hostages
The Iraq Islamic party has described the hostages as "activists in a movement opposing the war in Iraq".
The statement released on Saturday said: "The party calls on the captors to free them because their kidnapping is a dream opportunity for the supporters of the war against our country who say that Iraqis cannot tell the difference between those who support them and those who oppose them."